Doesn't surprise me. My agent went to pull my submission from Flux (we've heard from reputable sources that they have a horrible contract and don't really negotiate), and never got a response.
I've met a couple of their authors, who then went to other places. They were not impressed. That's Flux, not Llewellyn proper.
I'm working on my third book with them now, I've sold books to them as an agent. I wouldn't say they have a horrible contract at all, and I'm able to negotiate quite a bit of it.
As an author I've written for penguin and harlequin (in addition to flux) and as an agent have sold to them, S&S, Hyperion, St. Martins, etc, and there's nothing in their contract that is out of line with other pubs. They DO have royalties based on net, not retail, which isn't my favorite style, but there are favorable escalation clauses and bonuses that help balance it out a bit.
Further, for their advance size, I've been impressed by their publicity team and marketing campaigns. You WILL get a larger advance/print run at a big 6, but you'll get LESS marketing unless you're a lead title.
It's true that folks do often "move on" from there becuase the big 6 really does pay larger advances.