Organization, while it may seem straightforward, is a crucial component of building a healthy community. How effectively your community is organized can affect just how much new and existing members engage with one another.
Shortly: build an one room studio apartment, keep the door wide open and hope people come in and write something in that one and only guest book plus read all other messages too 
But yes, I totally agree the point of view where too many categories can be problematic. But only for the important 1 % minority that start topics. The rest 9 % of all users will answer it there is topics, no matter of amount of categories.
The structure of forum is really important, but amount of topics is the most important thing — and at new forum should use theirs time to write topics, not categories.
One, perhaps two, category AND 100 topics is a nicely structured and organized forum