I am surprised by what some people focus on when buying a library.
I listen to the demos, carefully, through good speakers and sometimes headphones. That's pretty much it.
I doubt that every single instrument has the same specs regarding legato, velocity layers, and all that. For any library company -- EW, 8dio, SF, Audiobro -- documenting all that and keeping it accurate enough to avoid fusses -- too much brain damage.
I keep buying from companies that put out libraries that sound great when I'm writing a whole piece. I don't really care how many layers there are. There are some libraries with tons of layers that, nevertheless, sound artificial when writing for a full orchestral or orchestral-plus piece.
So at this point I've had awesome results with a few companies, including Spitfire, EW, Spectrasonics, and a few others, and so I trust them and buy if the demo hits something I don't have.
The biggest advantage I'm thinking here is collaboration.