1. Many, many different types of exercise and sport will build an amazing bodies.... stop acting amazed that crossfitters have "decent size on them"
2. Time under tension isn't the be all and end all in muscle building and strength gains, if anything, crossfitters lift quite a bit more than your average gym-goer because the quick movements encourage a lot of power to develop through those movements.
3. Even with slower movements there is always the risk of injuries... speeding a movement up doesn't really increase this risk, it's poor conditioning altogether that leads to injuries (no warming up, muscular imbalances, poor recovery habits)
Crossfit isn't my cup of tea in terms of exercising right now, but I'm not going to hate on it until I try it.
Too many on this thread seem to have their heads so far up their bums to wanna try different types of exercise because their bro-science buddies told them a particular type of training is the be-all-end-all. Try being open minded for a change.