No. Training or therapy with low frequency electrical stimulation does not pose any danger whatsoever. This is because our muscles are continuously working with the help of natural electrical impulses all the time anyway. When you’re training with EMS, these impulses are simply amplified.
The major advantage to EMS training is that, unlike conventional training, there is no additional strain on the joints. The electric impulses work on the muscles directly, so there is no pressure on the joints ensuring they cannot be over-worked (as can often happen with weight training).