It isn't something they can't solve, it is more a problem of time. Like in fighting games, the developers have to go and test it out and tweak anything and everything that isn't fair. However, in KH, because multiplayer isn't the main focus, I find it often seems as if it is done somewhat lazily. I'm not saying it isn't great, all KH multiplayer things have been great in theory, but in actuality, I don't think any of them live up to the potential they could have.
KH's main point should always be single player so I understand the lack of focus, but I'd rather they only do something if they are going to do it well. Days, BbS, and DDD right off the start have problems with only being ad-hoc only without extra support (like Ad Hoc Party for the PS3). Second, Days had the problem of the characters being terribly balanced. Tiers were all to easy to make in that game, with Zexion sadly being on the bad end of that with Lexaeus and a few others. BbS had command balancing issues with Mega Flare when it came to hoards of enemies, Magnega and Aeroga too powerful against humans, and Curaga too good when dealing with humans because it healed too much health back. DDD on the last hand, well, Ryu Dragon is too good when flying with Dragon Breath, and Zolephant was too good with Rock Smash against computers.
Gripes are easily found in the multiplayer of the games, but I'm still thankful they implemented them. Everything has to start somewhere and I am very confident that KH will eventually get the multiplayer that it deserves