Anebel, it is good you brought those points up, because they are a reality when a horse has something new that restricts its neck movement.
Just the other weekend a fairly ignorant young rider put static (i.e. no elastic) vienna-style side reins on her horse. This horse has probably never been taught to accept rein pressure, and the rider is a very handsy rider. Anyways, she put these Vienna side reins on, got on her horse, and it flipped over and almost landed on her.
If a horse has never been lunged in a restrictive rein before, or hasn't been taught to accept rein pressure correctly, putting a restrictive device like side reins or a German Martingale on can make them freak out.
In my humble opinion, a horse should be taught to use itself properly, which will solve the problem that the martingale is trying to correct (ie. headset), rather than have a piece of equipment put on. There are much better, safer, more correct ways to achieve what the German Martingale tries to give you.