Is there a resource online which has made the effort to evaluate game user interfaces and estimate their usability (by genre)? (something like what Nielsen does). In-game reviews in magazines unfortunately do not always present an objective and quantifiable point, based on established heuristics or criteria
Short answer, no.
There however has been quite a bit of research into the topic (google "game ui evaluation").
The problem with this is its a very hard thing to measure, as a lot of it comes from the target audience of the game, or a players previous experience. Watch someone who's never gamed before playing a FPS and it'll be a world of difference to anyone who's gamed before.
That makes it near to impossible to pin down a universal set of criteria that makes a UI "good".
How helpful (or otherwise) the interface is tends to be a crucial aspect of the game, so it often falls within the domain of reviews. You might have some luck searching for "game name review interface" for instance.
Personally, I have a blog where I review a game occasionally, and I make a point of explaining what's wrong with the interface in detail when it's a big hindrance to my enjoyment of the game. I can't be the only one.