The progression of this game absolutely killed it from being actually good. You start the game and everything kills you immediately, but you can get to level 99 in like seconds with the starfalls at which point the game becomes incredibly easy. Bosses go from killing you in one hit, to dying in one hit. (exaggerating only slightly)
The final boss ended up being a step up from the one in the base game, but it was still ultimately a little underwhelming considering how much tedium it took to get there.
It's still Post-Update-2 Sonic Frontiers so it has all the same fun controls and high speed gameplay, but unlike the base game it couldn't carry the lesser design decisions around it. I raised my score of the base game from a 3/5 to a 4/5 because of how much updates 1 and 2 improved it, but this is just a poorly designed, low budget campaign with very little to offer, finishing Sonic Frontiers' upwards trajectory with a sharp nose dive back into mediocrity.
Cyberspace levels are pretty good though

Man… I still remember sitting at school and seeing the Twitter announcement for the 3 updates and now we are here, with all of them out. First of all props to Sega and Sonic Team for making these updates free, second of all The Final Horizon may have lingering problems from the main game like the pop-in and lack of cohesion in environments but the additions definetly make up for it; having ACTUAL different playable characters after 2 decades is incredible and aside from some weird moveset choices it all plays great, also it’s hard as shit but hey I’m not complaining. The ending for the game also got fixed and cyberspace was improved upon a lot, for the day we get a sequel to Frontiers I pray they do more of this but even better!

A lot of the content in The Final Horizon proves the team either didn’t have enough time or hardly tried with the base Sonic Frontiers. The buildup to the final act is much more fitting and the open world design takes a step up with much, much better platform challenges despite being very unappealing visually/ The new characters bring lots of fun to the table, and Sonic’s Cyberspace levels (despite being remixed versions of old levels) have incredibly fun design and prove how they can actually design really fun levels when they actually try. It unfortunately begins to falter towards the very end with a super tedious and poorly designed boss rush, and a final boss which has one of the worst cases of the game not properly teaching its mechanics I have ever seen. However, The Final Horizon’s content proves to be a big step up from the rest of the game, however this is still only about 25% of the whole Sonic Frontiers experience.