Armor Systems

Couldn't you fix this by increasing the damage of the bulldog? Maybe also decrease it ROF? Besides, the bulldog's purpose was to absorb damage for ARM's abundant "high damage/low hp" units, not necessarily to deal damage itself. Also, it's not that the bulldog COULDN'T be made useful using TA's system, it just that it wasn't at the time of TA's demise (only referring to "official" support here; I don't have enough experience with spring games).

Same answer as before - the average battle in PA could involve more units than the max unit cap of StarCraft. The more information there is, the more complex it becomes and the less units you can process it on. StarCraft is a small, tactical game.

Turret turn rate, projectile speed etc. are all stats that StarCraft did not have, because it didn't simulate physics. This means you don't have to use "hard" stats like armour to balance units or create the unit relationships you want, and these naturally occurring relationships are far easier to understand than arbitrary ones like armour, because they follow rules. A fast, light unit can be made good against a heavy unit simply because the heavy unit has a hard time hitting it, or because the light unit can kite it with longer range. That's easy for a player to see and understand. What's not easy is having to learn how much damage a unit does to each other unit, because it varies on all of them due to armour.

I'm a long-standing supporter of low-level unit physics, like CAE-style simulation of unit's internal parts, their interaction and physical effect of damage on unit's functionality. That, however, is insanely slow, and so far I've seen implementation only for relatively simple reactive propulsion in a game called Kerbal Space Program, which is definately not a planet-scale RTS, not even an RTS at all. In KSP, simulation was made for a single rocket-like craft, and even so it wasn't fast for complex parts combinations. Proportional directional armor is in some way closer to that than other variants, so I'd go for it. However, if it will hamper performance, I don't mind to stick with plain old HP.