I think the lore is very mixed on it since even though people love the EU it basically meant every author sort of did different things with it. I believe a common thing with the armor is it does stop the blaster rounds but would make the user's incapacitated. Copout I'm aware but we also have to remember that stormtroopers were always meant to be the elite and most dangerous force but due to them going against people with plot armor they couldn't be good at all. So we have a generations of people who sort of internalized that and basically continued the meme that stormtroopers can't hit the broadside of a barn (even though they were pretty effective in 1 the opening scene and 2 with the rebel base).