As pointed out previously, there are many concepts for what power armor does, but for the version in the game it does not “free you from any concerns”, it “makes it possible to use gear heavier than what a human is capable of”.
It’s military-grade yes, but the military isn’t going to care that much about melee, the encumberance it applies shouldn’t affect shooting, because that’s what it’s going to be designed for, but running/walking/melee and many other actions are going to be impaired because they don’t matter for a soldier.
Radiation (fallout aside) moves in straight lines, gasses can move around corners and through gaps. Bulk/weight aside, it’s far easier to shield against radiation than against toxic gasses.