Apart from the newer interface, firefly is not very good.  It can`t render double sided polygons or objects with reversed normals almost every other 3d renderer can render these ok.

Some of the other newer things are not so good, pmd files are a nuisance and I have noticed that they are being used much more now, even with things that don`t need them.

For figures and objects that don`t have a lot of (or any) morphs a pmd file is a waste of time.

These are not the only bad things about the newer versions, but they are the only ones I can think of at the moment.