what do you mean by serious?
As one of my instructors put it, "this isn't knitting class". So I would agree that a certain amount of bumps and bruises are to be expected.
Having said that, I would not train in an environment where it would be "expected" that I would suffer an injury that would keep me off the mat for two weeks or longer. There certainly are risks for any physical activity, and certainly risks involved in aikido, but to my way of thinking a training environment should be constructed to minimize any serious injury. There are certainly enough modern equipment to make such an environment, IMO (ie crash mats, general physical conditioning, protective gear --- like mouth guards, cups, and good old common sense)
While I have seen serious injuries in aikido, bjj and judo, they were all completely unexpected and of the "unfortunate once in a lifetime" event.