Are they difficult to master?
It's straight forward to follow the instructions and after that it's about practising to get the feel of the machine.
However, like any machine, the real issue is to make an effort to learn enough about how to get things right when they don't go to plan.
There are two main issues with using any coverstitch machine-
- managing to keep the long foot level so that it stitches properly over seam allowance bumps and lumps
and
the technique for releasing the and pulling through the threads at the end of a run
Several tutorials around.