The advance they offered me was less than a quarter of the advance I've seen quoted for other specialised tech books, and about a tenth of what an author would expect for a best-selling mainstream consumer-oriented title.

They also spam people on mailing lists looking for authors. (Not spam the mailing lists - harvest addresses and contact people more or less at random.)

I think this low remuneration affects the quality of the authors they are able to attract, so you're right in that part.

From what I've seen, not all the books are terrible. ;)

But they're certainly not in the same league as No Starch, or most of the self-pubbed titles on LeanPub.