I think I remember reading something somewhere about judo-related concussions causing short-term memory loss, but I have no idea what it said or where I read it.
Under the assumption that you are going to an average recreational judo club. Safety is normally the main issue. People have jobs and don't appreciate getting injured or getting marks on their faces. Now, if you are going to train at an elite competitive dojo. Something like International Budo University in Japan. It's dog eat dog and stuff like that might happen.
One thing you should ask if they are training on a spring loaded floor underneath the tatamis. That cushions the impact a lot and reduces possiblity of injury. Tatami mats on top of cement floor increases the risk of injuries.
i messed up one night and didnt take a break when i needed to. i was way to tired and got sloppy. i took a throw and didn't do a basic rule of ukemi (look at your belt), and took a nasty bump and roll on my neck. if i had done what i was supposed to do, i wouldn't have had that very mild injury (yes, it was a stiff/sore neck for 3 days and i was fine).
i've also gotten chucked off the mats twice. both times i did proper ukemi. it didn't feel good, but proper ukemi and i was uninjured.
****, i let one of my colleagues throw me with an o goshi on a matless, carpeted floor just to show my students that breakfalls work.