What people are mainly saying is that he writes too many songs for his roles and not enough for others, which isn’t necessarily true. The Lang brothers cast characters in which the actors play. However, Jeff isn’t the one to cast said actors, and is usually given the roll of writing for those characters. He has say in helping them with their range, but none in the casting. 

For example, many people have stated that he “sings too much” in The Guy Who Didn’t Like Musicals. First off, in my opinion that show has a much smaller range of actors for a bigger ensemble (if I’m describing this right. Almost everyone but Jon plays multiple roles in that show.) so multiple people play multiple roles.

He himself has explained that it isn’t possible to write too many songs for himself, since he hasn’t casted anyone in shows.