Yes they can be drilled.
Advantages? Speed, possibly cost too if the person making them is slow.
If your bathroom fitter is tiling also, then making his own can allow him to line up tile joints and fixtures to suit the niche and vice-versa.
If he isn't used to making them, then watch out! They are a potential water-trap so need 100% care with the waterproofing.
Cost-wise I would charge similar to make them, assuming a standard stud wall construction.