Depends on the plane. The upper planes are largely benign, but still have minor dangers, and are dangerous if you go looking for trouble most certainly. Limbo you're likely to drown/petrify/boil/roast just from the environment, to say nothing of hungry slaadi. Mechanus has modrons that may march over you uncaringly simply because avoiding you wasn't one of their daily orders. The lower planes are, in a word, hellish.
My storyhour has been steadily building up towards open warfare on Oinos, the first layer of the Gray Waste. When I get to that in the next update or two, I'll be detailing a portion of it from the perspective of a mortal mercenary in the Blood War. Nothing mortal survives the War Eternal intact and unchanged, usually for the worse.