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.
Of course, that comes at the expense of nuking your own front line, who in OS2 are much less likely to have high resists or magic armor than they did in OS1 (where you could get 100% resist fairly easily).
Since magic damage is almost entirely AOE based you end up "wasting" it on high magic armor targets whether you want to or not, and it just so happens that low-magic-armor targets tend to be front liners, meaning they want to be standing right next to you and your own low-magic-armor characters.
Personally I ended up going through the game without any characters dedicated to magic damage, and my second playthrough looks to be about the same. AOE damage just doesn't work so well for breaking armors efficiently, which is the most important thing as once armor is broken you can stop an enemy from ever taking another turn. Also physical hard CC is much easier to apply anyway. Frozen and Stunned both take a two-part combo (must either be shocked/chilled twice or wet), wheras Knockdown works just as well (better, actually) and can be applied safely with non-friendly-fire AOEs in a single hit.