Two main issues. First, the genre combo has no synergy. 2D Metroidvanias have a lot of backtracking so the combat should be quick. Turn based monster collectors tend to have long battles so different play styles, such as set up or stall, can be viable. Likewise, in monster collectors the player doesn't have access to every ability they've gotten at all times, one has to choose what team to bring. This opposes the Metroidvania idea of trying all your new abilities when you are backtracking. This genre combo was just a bad idea from the inception.
Second, the monster collecting gameplay encourages constant switching but makes it annoying to do so. This is just a problem with the developer's execution. In games like Pokemon there is a weak incentive to switch monsters as using the ones from the start can easily take one through the whole game, leading many players to just use their one Charizard as much as possible. This compliments the highly detailed level of customization though EVs, IVs, moves, abilities, held items, etc. Oppositely, games like Megami Tensei give a strong incentive to consistently switch demons in your party, as each demon will stop learning moves after only a few level ups. There is relatively little customization so fusing away a Jack Frost you've had for an hour doesn't feel bad at all.
This game can't decide what camp it wants to be. New monsters are easy to acquire. Hatched eggs will be at your party level so they can be useful right away. However, there is also a large amount of customization. There's skill trees, held items, form differences, and having to set these up every time you get a new monster is very tedious. The Metroidvania aspect asks the player to use the latest monsters for overworld abilities, but the turn based combat asks one to get out their spreadsheet to figure out the best investment for skill points.
Overall, this game had a bad concept and has bad execution. I imagine I enjoyed this game much less than others because I am a huge monster collecting game fan, and thus have played all the notable works of this genre. However, likely many of the fans of this game were people who only played Pokemon, were disappointed enough by Sword/Shield to finally try a different game in the genre, and then loved this because it was something different at all. Releasing a monster collecting game in 2020 was a fantastic release time.
Second, the monster collecting gameplay encourages constant switching but makes it annoying to do so. This is just a problem with the developer's execution. In games like Pokemon there is a weak incentive to switch monsters as using the ones from the start can easily take one through the whole game, leading many players to just use their one Charizard as much as possible. This compliments the highly detailed level of customization though EVs, IVs, moves, abilities, held items, etc. Oppositely, games like Megami Tensei give a strong incentive to consistently switch demons in your party, as each demon will stop learning moves after only a few level ups. There is relatively little customization so fusing away a Jack Frost you've had for an hour doesn't feel bad at all.
This game can't decide what camp it wants to be. New monsters are easy to acquire. Hatched eggs will be at your party level so they can be useful right away. However, there is also a large amount of customization. There's skill trees, held items, form differences, and having to set these up every time you get a new monster is very tedious. The Metroidvania aspect asks the player to use the latest monsters for overworld abilities, but the turn based combat asks one to get out their spreadsheet to figure out the best investment for skill points.
Overall, this game had a bad concept and has bad execution. I imagine I enjoyed this game much less than others because I am a huge monster collecting game fan, and thus have played all the notable works of this genre. However, likely many of the fans of this game were people who only played Pokemon, were disappointed enough by Sword/Shield to finally try a different game in the genre, and then loved this because it was something different at all. Releasing a monster collecting game in 2020 was a fantastic release time.