Actually it is very hard to get the leg of an experienced player. If you miss you are almost always in a very bad place.
Shime waza (choking techniques) are actually very safe under the proper condiitons. The Kodokan did a large study (Dr. Kowai I Believe) and could only find one death from choking, and that was not in a Judo class or event. It was at a self-defence school.
Most places ban heel hooks simply because if the defender makes an error, and turns the wrong way, he can be injured very badly. The saying in Shingitai is "We all have to go to work on Monday."
The real key in any submission sport is that the opponent must be given the time and opportunity to submit. Its why we don't let you throw with a locked arm for one example.