It's quite easy to nick with any electric shears with combs/ cutters, you have to set the blade/comb distances correctly so there's no over cut and the distance between the bevel on the comb and cutter blade is correct.
There are specific blows to use when Crutching that minimise the chance of hitting the tendons etc, also if you do choose this route, never pull the wool up to get the comb in, always keep the comb on the skin flat and pull the skin behind the comb back so the wool is pulled onto the blade if that makes sense, keep your left hand over any sensitive parts.
If you arn't comfortable with turning the sheep over and keeping her still I'd probably use hand shears, the shearing hand piece is a bit more vicious than your standard clippers and you have the opportunity to do a lot of damage very quickly
British wool board I think do a Crutching course? Certainly there are Crutching courses at agricultural colleges