How many engines are MU'd in switching locomotive consists, gensets or otherwise, is usually a function of tonnages to be handled and the gradients to be encountered. In switching operations that can vary day to day, so I do know of cases where such consists had more units than might be required.

It should be noted that missions can change, with the same consist used for different jobs. One job may be using the consist to switch, while another job may be using that same consist for a "hauler", which may have more cars/higher tonnages, thus requiring more power. Also, many Gensets I've seen generally have a lower horsepower rating, compared to work horses like the old SW 1500's, often requiring mu'd sets to do what a single higher horse powered "conventional" switch engine could do.

I haven't heard a lot of comments from folks I know about reliability issues with the current generation of gensets, although all loco's have failures from time to time. Those who have operated gensets more recently may want to chime in on that, though.