The core problem with armor abilities is that they don't work in loadouts. Loadouts either make them horribly powerful or restrict their potential. Think about camo, for example, it was a gret power-up, no one complained. It was something you fought for, something you wanted to get. It had no restrictions such as jammed radar and making you more visible when moving. It was perfect. Then, loadouts came, now it's the camper's dream because A) any kind of movement with it makes you visible which forces you to either move slowly or stay stationary, B) you can activate in anywhere you want and get it stright off spawn.
Then, thinking about what the armor abilities that previously weren't power-ups could be as power-ups. Promethean Vision: you pick it up, get immediately a certain amount of time where you can see through walls. However, you have to utilize this ability fast as it starts immediately on pick-up and the drain cannot be stopped. Huge advantage when used skillfully, a nice little advantage in regular gameplay, just like overshield and active camo.
Jetpack also was an ability that had the problem that a player could get it anytime they wanted and every player could have one. Were Jetpack a map pick-up, it would provide an advantage, but not one that would completely break the concept of map control when only one player has it.
Armor abilities aren't the problem, they're a good idea. The problem is restricting their gameplay potential by adding them to loadouts and messing everything up, which leads to situations such as the case with Jetpack in Reach where carefully crafted map control spots were completely irrelevant and it didn't really matter were you on the top, or the bottom, you could still be beaten by Jetpack. Alternatively, there could be the horrible maps like Swordbase that were so horizontal that top control was way too easy.