So, iv'e been testing out party members and found some possibly helpful information.
For whatever reason, when you set a party member to "finish the job", they will attack twice as much then normal. For whatever reason party member ai is complete garbo by default, party members have a terrible turning circle and for whatever reason they circle heartless for up to 10 seconds before actually attempting to attack, it seems there is an invisible timer preventing them from using any basic attacks, but an oversight in coding (possibly?) almost entirely fixes party members simply by setting them to "finish the job". It's still not perfect, they'll still circle enemies every now and then, but they seemingly attack more than twice as often.
I still need to test some party members, but from the four i tried it had the same exact affect.
Unrelated, but some other stuff i found.
Donald- On uneven ground donald's fire magic has a high chance of clipping the ground and instantly disappearing. Donald's ai seems to have some sort of basic "if there is an object in between me and the enemy don't cast fire/blizzard" function, but it's pretty unreliable, he'll often move only slightly and clip the side of a tree or column with his magic anyways. On the plus, donald seems to have a good understanding of enemy weaknesses (he won't cast fire on fire cores for example). An unfortunate oversight however, On the off chance you find an enemy who takes reduced damage from magic in general (which is rare). Donald will completely refuse to do anything other than simple melee attacks.
Take the sorcerer in the twilight town battle gate for example, the sorcerer takes slightly reduced damage from all magic and because of this donald's ai thinks the best course of action is to jump at it and whack it when it's close enough to the ground. This results in donald standing underneath the sorcerer in his idle animation for most of the fight. As for donald's moveset, he has a two hit combo and a basic single hit air attack, both of which are pretty bad in damage output. His magic, while very weak in comparison to your's, Is what makes him pretty decent. Overall, donald's biggest weakness is his ai, and his biggest strength is his magic, no surprise really.
Goofy- Like in the previous games, goofy naturally does contact damage while moving forward and blocks any attacks to the face. Goofy's tornado move is weirdly inconsistent when it comes to duration, sometimes it'll only happen for a second and sometimes for several seconds. His multihit shield toss move now does one single powerful hit instead. Only other notable thing's to talk about is his basic attacks. He has a single hit combo (maybe two? kinda hard to tell honestly) and a single powerful air attack. Overall goofy does a good amount of damage, and is a generally more consistent damage doer. He's pretty equal in power to donald honestly.
Hercules- Hercules is (from the few i tested so far) one of the best party member's the series has had. He only has one glaring issue, but he makes up for it in other areas. Most (if not all) of hercules attacks do massive damage. He has weirdly two different kinds of basic attacks, There is his simple punch, which is slow (but decent in power) and his charging move, where he charges headfirst into enemies. While the punch is okay, the fact that he can use this charging move without having to use mp is a big plus given how powerful it is, it can hit multiple enemies and he can turn while using it to steer towards them.
This is where we run into a glaring issue however, and that is his complete lack of an air attack. Fortunately he makes up for this in one of his special moves, that being the rock toss. His rock throwing move is excellent for mainly one reason. Super-armor. While he can take damage while starting up the move, he won't get knocked out of it, he'll see the move through, that rock is gonna get thrown! The spinning part of the animation also does damage.
This is his main way of attacking air enemies and it's damage output is decent, usually killing whatever it hits. Unfortunately, if his target dies before the rock is thrown it's almost guaranteed that he'll throw the rock at nothing, rather than looking for a new target. Hercules' best move by far is his energy explosion. During the startup, any enemy that get's too close is sucked in and massive damage is done with the explosion. This move cancels out pretty much everything, even moves where an enemy would normally be invincible, they're taken out of the invincible state and sucked in. Doesn't matter if the enemy in question is charging him at light speed, once it enters the attacks range they completely cease all motor function. Overall hercules is what all other party member's should strive to be. All of his weaknesses are covered. 'nuff said.
Woody- Woody is the last character i tested, and he still needs some testing honestly. So far though he seems to be a much weaker version of hercules. Something that i fear might become a pattern is the fact that it seems woody also lacks an air attack. I hope donald and goofy aren't the only ones with air attacks, but i fear that may end up being the case. Woody's best quality by far is his multihit combo, which, if all attacks connect does a bit more damage then hercules' punch attack.
Woody also has a much weaker version of hercules' charge attack, except it has less range, and he can't turn while using it. And it costs mp... Honestly pretty bad attack. His lasso attack isn't much better. It acts just like hercules' rock toss except it's range is pathetic in comparison, as is the damage output. It doesn't take as long to actually throw as hercules' boulder does, but it's lack of super armor makes it pretty bad. Woody still need's some testing, but so far, he only just beats ping in kh2 in usefulness.
I'll probably make some sort of party member tier list later, these are just the results i have currently.