Re: Hard Floor
I practiced for two years in a UK dojo with no mats. The floor was wooden but was upstairs and had some give as the room shook. The first night I went there (after 3 years on mats) I thought they were joking...
We practiced few foreword rolls, no break-falls, but all the usual techniques. I have no doubt that some mat-less practice improves both ukemi and nage waza. The latter through the more obvious connection to the floor and the feeling of pushing off the big toe. It also slows practice to a point where you can feel things through. The former through survival.
At the dojo we found that something was missing however and started training on mats once a week. Also worth noting is that students there, who have not trained elsewhere, think it's no big deal as they don't know any different (see also Sorinji Kempo).
Re injuries - I saw less injuries there than matted dojos - though long term I would watch the knees, and for those making a sudden change to mat-less there are definite risks.
Personally now, I'd recommend a spell of trying it, but I like the aerobic expansiveness that mats can provide - they're a nice privilege.