For all intents and purposes, the game was marketed as a turn based RPG with a combat system that actually required conscious input and thought in order to play. While this is technically true since there's no auto-play button like many JRPGs have, the combat turns mindless pretty quick.
Start of the game before the death of the demon king is somewhat challenging. Instead of the usual turn based system, every character in your party has a number of attacks they can do per "turn," which they can use at any time and all recharge independently of each other. As long as your character has an action pip and the enemy isn't currently attacking, that character can make an attack. Blocking enemy attacks at this stage only mitigates damage (with better timing lessening the damage moreso), you have about two attacks per character, one bar of super meter to do special character specific attacks (which also drains if you hold the block button, encouraging you to time your blocks carefully) and your health is relatively low, so you have to plan out your attacks, execute them at the right time and have a healer at the ready to fix you up. All elements of the combat at this stage come together.
After the demon king dies, however, you inexplicably get a massive stat increase. Where you once had one bar of super, you now have three, and your health has gone from about 300-500 to somewhere in the multiple thousands. Enemies do a bit more damage, but as long as you block it's nothing really meaningful. This stat increase happens twice more throughout the game, and it also increases your damage output (you still level with experience from enemy encounters, so you're also dealing much more damage by the end that way).
As you go along, you slowly unlock more characters, but there are some who are head and shoulders above the rest. As a quick primer, most characters deal physical damage, while there are some who deal magic damage. Physical damage hurts enemies more, while magic damage generates more super meter per hit. However, there is rarely a need to generate super meter more quickly, as you already get a good amount from attack spam and you hold onto it between encounters, so magic damage is mostly just a damage handicap. Further, there are enemies later in the game that actually heal from magic damage, making magic based characters useless against them. There is no such counterpart for physical damage, and no apparent resistances to physical damage. In earlier builds, magic damage used to be able to go past enemy defenses for free, but that has been removed for release. So there really is no benefit to using magic damage.
So, now you're left with physical damage characters, essentially. Even among them there are particular standouts. Two characters in particular, Tungar and Leilani, both have AoE attacks that deal massive damage to all enemies around them, and generate a ridiculous amount of meter from using them. By late game these two characters in conjunction with attack upgrades will shred most encounters before anyone else gets to have a turn, your other party members included.
Further, the upgrades in this game completely break the combat system. You collect red gems throughout the game that you spend to upgrade your attack or your defense. Upgrading your attack gives you more actions per character, to a maximum of five a "turn." This gives the already superior characters more attacks to lay the hurt on your enemies, so many combats by mid-late game are over before the enemy even gets to attack, some boss fights included. This is coupled with the defensive upgrades, which lessen how much damage you take when guarding, increase the window where you can land a "perfect" block, and also make it so "perfect" blocks heal damage when you land them instead of further decreasing damage. These defense upgrades effectively make every character their own personal healer, so all characters who have a support focus are rendered useless. I have not heard of a single late game team composition that has a dedicated healer in it.
So, by endgame, you have a whole damage type out of two you've been conditioned not to use, support characters who end up being a wasted slot in your team comp, and two characters that are the spin-to-win button. What appeared to be a deep and challenging combat system on the surface is revealed to be a broken, unbalanced mess.
And, as mentioned, they scrapped many planned combat mechanics, and some of the characters are just unfinished (two of them so unfinished they're not even in the game as of posting).