while in theory everyone can use everything, using plate on mages, rangers etc is fucking awful since heavy armor reduces their action speed (by increasing recovery time - naked mage takes 3s time to cast and 3s to recover, while mage in heavy armor takes 3s to cast and 6s to recover)
same with melee weapons on classes which don't excel in fighting in close range - for example wizard has shitty melee accuracy, sure you can give him a poleaxe or whatever but don't expect him to be useful with that
and yeah you can make a terrible party, a team without a frontliner and a healer is gonna be pretty bad (at least compared to a team which does have them).
overall the class design is okay (..ish. theres not a lot of customization). the attribute and skill system is pretty much cosmetic though. the attributes have little to no real impact (there are no heavy penalties for dumping a stat, and no very strong benefits for maxing a stat, so you can just assign points more or less at random and its gonna be fine), and the skills, well, there are like 6 skills, and with 6 characters in the party you can basically master everything, so the skills might as well not exist at all. it feels like both these systems were added "CUZ GROGNARDS" because they don't really add anything.