It is unsafe if the rider isn't well balanced. A Dressage saddle has straight flaps on it, as opposed to the forward cut with knee rolls on a jumping saddle. The reason a jumping saddle is forward cut, is to allow the rider to shorten the stirrups and use the leg to balance instead of the seat. This allows the rider to get out of the saddle and 'fold' over jumps.
In a Dressage saddle, you can't yank up your stirrups because your knees will go over the flap. Then you have no support for your leg, and therefore your balance is going to be severely effected.
For people who are extremely well balanced and are able to ride with Dressage length stirrups over a jump, then this is not an issue. For someone who really needs to use their leg for balance over jumps, they are going to come unstuck. That's where the 'unsafe' aspect comes in.
If you just want to pop over a few little jumps, a Dressage saddle will be fine. If you're wanting to jump bigger, then you're best to invest in a umping saddle. I wouldn't buy a jump saddle to compete in Dressage, and a jump rider wouldn't buy a Dressage saddle to compete in jumping.