FFXVI is an incredibly frustrating game because there are moments where it CAN be really good but it's so insistent on dragging itself down at every opportunity. I wish my problems with it were as simple as "it's an action game and not an rpg".

At best the "RPG elements" of FF16 are ultimately meaningless and feel like going through the motions. Sure, as you progress the numbers get bigger. Once in a while you go to the blacksmith to craft new weapons or armor, that sometimes you use for exactly one battle before better ones turn up, that make the numbers bigger but have zero meaningful effects on combat. There's ostensibly a crafting system but it will never ask more of you than maybe doing a side quest or hunt to get a key item as you'll never be at risk of running out of regular crafting items.
At its worst the rpg side actively makes the game worse. So much of the game is comprised of doing "quests" where you warp to the closest fast travel point, follow an objective marker through a lifeless world, and occasionally fight one of the small handful of repeated mob types. There really is nothing to do in the world besides following objective markers; you will never come across something new organically as every side mission is marked on your map ahead of time (outside of hunts) and every time you go out of your way your grab a bit of treasure it will without fail be yet more shit crafting material or an absurdly small amount of gil. The hunts will spawn in the world as soon as they're available on the hunt board and checking the hunt board doesn't queue an objective marker, so there is some small amount of exploration and potential discovery when dealing with them - not much as the hunt board will typically tell you directly where they are, but it's something. You have party members, but besides Torgal you have zero influence on their actions and they mostly serve to just steal kills.

The action side of this action rpg is pretty underwhelming too. Your basic moveset is fairly limited with just a simple four swing string on the ground and the air, a stinger and helm splitter, a dodge, a charged sword attack and chargeable projectile magic. You can time magic presses between swings in your string, which can give the impression of complexity but doesn't really lead into much besides tacking on more damage or charging your projectile. The aerial combat is weak overall with pretty limited options for actually staying in the air and a very awkward to use launcher in Torgal. You get a devil trigger partway through the game too, though it's about as basic as they get dealing more damage and healing while it's active.
The Eikon abilities and attacks are the big new ideas and they're also a pretty mixed bag. The problem with the abilities is how unbalanced they are compared to each other. Utility abilities like Garuda's Nero snatch grab and Titan's royal guard block/parry are far and away the best abilities to have on you, while I found the chargeable attack abilities like Ramuh's BbS shotlock and whatever Bahamut's got going on are borderline useless since the time it takes to charge could be far better used.
The Eikon attacks have everybody's favorite modern action game trope - arbitrary cooldowns! These are usually at least a bit interesting and it can be a bit of fun to mix and match them once you upgrade them to be used when any Eikon is equipped, tho they largely fall into attacks that do big damage or attacks that do big stagger damage.
Eikons also change your magic's color. There's no elemental weaknesses or anything despite the game revolving around elemental beings so this is an entirely cosmetic change.
The biggest problem with the Eikon system is the distribution throughout the game. Throughout the first half of the game you only have access to Phoenix and Garuda and you only get Bahamut, Shiva, and Odin in the last fifth of the game - meaning for the vast majority of the game I just had Phoenix, Garuda, and Titan equipped with little incentive to switch them around until I got Shiva (as its ability is almost a straight upgrade on Phoenix's) and then Odin (by far the most mechanically interesting Eikon) barely an hour or two later, at which point the game is almost over. There's nothing wrong with saving the coolest abilities for near the end of the game to carry over into NG+ of course, but this much of the ability set and at the end of a full size RPG instead of a shorter action game is pretty egregious.

The real killer of the combat is just how lacking the enemy design and variety are. Virtually every fight can be boiled down to cycling through your Eikon attacks to tear through mobs or using your Eikon attacks that tear through the stagger bar of bigger enemies while dodging and counterattacking then using all your big damage Eikon attacks when it's staggered. Rinse and repeat for roughly 50 hours. Because of how limited the combat mechanics are practically nothing changes about the enemies as they get reskinned and reused throughout the game because practically nothing can change - you can't tweak what kind of damage they do or can take like reskinned enemies in most other rpgs when that's not even a factor. The hunts are largely reskins of enemies and minibosses from the main game as well, just a bit tankier.
The major bosses still rely on roughly the same gameplay loop as every other enemy with a stagger bar but do throw out slightly harder to dodge strings. However there is a huge over-reliance on MMO-style AoE attacks where the ground will light up in a pattern and you have a few seconds to stand in the right spot to avoid damage - this just isn't very compelling action game boss design. I'm not a FF14 or MMO in general player so I might be completely off base on this, but along with the tedious quest design and constantly cycling through cooldowns this is where it feels most like this is an MMO dev team trying to apply what they know and what works in an MMO to a traditional action game and it just really doesn't work.

I've been extremely harsh so far but it's all important to understanding what I do like about FF16 - when it cuts out the bullshit and embraces its true calling as a big money dumb action game powered by sheer maximalist presentation.
The dedicated action stages don't fix the core limitations of the combat by any means, but they are much more fun than the standard encounters found through quests and in the world. While I've detailed why I think the combat does not hold up across an RPG length and framework it IS very fun to smack enemies around with big fuck off moves in an environment that has a consistent forward momentum and pace.
But what really saves the game are the Eikon battles. Massive kaiju on kaiju action setpieces where you can see that Square Enix mainline Final Fantasy big budget come in with a level of scale not seen since the likes of Asura's Wrath or God of War 3. Fundamentally these don't change much about the combat but the sheer presentational hype is enough to carry them into being by far the highlights of the game. It would never fly with the expectations of AAA games today, but there is perhaps another timeline where FF16 was a 12-18 hours long straightforward action game and while it wouldn't stack up to the depths of the Devil May Crys and Ninja Gaidens of the world it would be much better for it.

I've focused largely on the gameplay but I want to touch on the story a bit too. Overall it's... ok. The sheer presentation of the big story beats carry it into being hype at parts but it doesn't really tread any new ground and the parts where it's trying to be a serious medieval fantasy story are frankly kind of embarrassing. The worldbuilding and slavery analogue feel pretty sloppy, since you're teleporting all over the place to do shit quests and people will just show up places as the story requires any story beat that relies on "it's going to be a long journey" falls flat, because of that aforementioned issue and how barren the world is in gameplay the world feels so tiny, and any scene with Anabella or Benedikta is almost shocking in how juvenile they feel. (like I'm gonna keep it honest with you FF16 it's kinda weird that 2 out of 4 of your major female characters are evil sexy women who use sex to be evil because they're insecure). The English writing can also reallyyy feel like it's trying too hard sometimes and every time a quest name or item description has a "cheeky" joke in it I want to gag. I don't play FF14 so I don't want to make any sweeping generalizations about Koji Fox's localization style but I can't say I'm a fan of it here. FF16 clearly wants to remind you of Ivalice but the worldbuilding and writing are a far cry from the likes of Tactics or FF12.
The game is absolutely gorgeous though - while combat can occasionally have readability issues because of all the blinding HDR particle effects flying around the cutscenes could have easily passed as the big budget pre-renders RPGs had ten years ago. I opted to play the game in its quality mode - mostly because the dynamic resolution in the performance mode turns the game into a blurry mess when battles start and while it isn't as smooth the quality mode feels fine to play - so I got to enjoy the shiny graphics even more. I gotta say tho - Clive's outfit is kind of ass and the actual designs of the Eikons are pretty boring.
The music is pretty good, it does blend together for me a bit but when it steps out of its orchestral comfort zone it can really shine - I wish more tracks were as out there as Typhon or Titan Lost's themes.

This was a lot of text to basically repeat what everyone else has said - FF16 is a game of high highs and low lows. Unfortunately more lows than highs but I do truly think the highs are worth enough to push through the pretty dire low points.