I've been surprised at times by what's missing. There are are some strange omissions, from most recent memory around the Bourne shell builtin commands and make(1). I've had to go hunting in other BSD distribution manuals at times to find what I need. I'd say the GNU core utilities sometimes have better docs for their equivalent commands.

That said, for non-core utilities on Linux it's pretty hit-or-miss. The BSDs are generally pretty consistent in what they do offer, and that's what I love about them. Of course it's a different development model and it shows.