I have a Yamaha silent (nylon string) guitar. Although not a bowed instrument, the issues will largely be the same. It is great fun to play and I am happy to own it.
The most noticeable difference when compared to an acoustic instrument is in its treatment of dynamics. When wearing headphones, the range of dynamics is much narrower than one who realise with a natural acoustic instrument. I find myself having to greatly exaggerate the velocity with which I pluck a string in order to obtain the same range of dynamics. These issues are the result of the electronics and effects included with the instrument and the result can be rather grating on the ear, especially when playing more sensitive phrases with intended pianissimo. When one is a learner, it is tempting to use the built in effects to compensate for inadequacies elsewhere.
This problem is further compounded if one plays the instrument without the headphones. Playing without the headphones produces volumes similar to an unamplified electric guitar and the dynamics are almost non-existent. It's easy to get lazy and simply pick up the instrument without the headphones. In fact, this is the most frequent way I play it. If I learn a piece in this manner, when I pick up an acoustic instrument I routinely pluck the string with far to much velocity.
So I would say that it is probably not the best way to learn an instrument, even if it is great fun to play with when you have a bit of experience. Also, the tone is obviously much different.