Supporting variable-width fonts is pretty easy on its own (if bit-packing, you might wish to store them vertically), but does complicate combining characters (remember, they go in different places and may need to make the new character larger).

For vector fonts, the interesting question is "if I render this at a different resolution and then scale it, will it line up?" which fundamentally has no answer that can satisfy everyone. Most other difficulties are merely a Small Matter of Coding (and providing appropriate APIs).