Why not to isolate squats and jumps? If you are concerned about injuries, break down the whole thing into components and do it separately. Squat jumps require perfect balance and technique to implement. Also, the shoe you wear and the floor on which you do them make big difference. Regarding how safe it is, it depends on how you do it. People have been doing it for years and have no problem, while you may stuff things up on the first go. Safety is the first things you should take into consideration. If you have any doubts in mind, don't do it. Half-heated and doubtful approach won't take you too far. Having said that, if you separate squats and jumps, learn the right ways of doing them. Risk is always there in everything, but with proper form and technique, those risks can be minimized, or avoided.