I'll say that I've accumulated a lot more injuries in aikido than in other martial arts. I think it's, perhaps, due to the fact that we actually complete the throw in aikido whereas, as BC said, we tend to pull our punches and such in karate and other punching/kicking arts.
As far as falling on hardwood floors and such, I feel that everyone should be able to perform rolls comfortably on them. Being able to breakfall on hard floors is nothing spectacular, really. if I do a regular slapping breakfall, my hand and arm stings for a bit; if I do a "soft" breakfall (like you may see with some Yoshinkan people), it's not that uncomfortable at all.