The armor system essentially removes a huge component of RNG in the combat system and shifts it to a more tactical approach on every turn.
As for mixed party, there's advantages and disadvantages. One advantage, obviously, is the ability to ignore one armor type entirely. However, you never have a say in what armor type you want to whittle down. If that one guy has 500 armor and you're full physical, you're going to have to deal with that 500 armor first. A mixed party, however, can split into two and effeciently have the casters take down the front line and the physical damage dealers hit the mages.
Each approach has different tactical value and, in my experience, depends on preference.
I, personally, prefer a mixed party. In addition, there's ways to mitigate the disadvantage of a mixed party. Locust swarm, corrosive touch(if you can manage to get it, recipe currently bugged), touch of decay, shield lob are all excellent ways of dealing physical damage that scales well for a caster. Medusa head, marksman's fang, bull rush are all ways a physical damage dealer can assist the casters and bypass physical armor.