They are not composable. If I use yo webapp and make some progress, I can't later do yo angular or yo requirejs
This also leads to a bunch of out of date generators. If the "yeoman style" changes I can't be certain that all the generators that are out there have updated as well.
Generators should be additive. yo angular should depend on yo webapp, for example. This way when updates are made to the webapp generator then the angular generator (or any generator that builds on webapp) will also get them.