Speaking as a long-time fan who just finished a marathon read of every MWDA novel, I'll go against the tide. I'll say that the badness of the Proving Grounds series, while underniable, has also been massively played up over time. They're bad, yes, but they're far from the worst BT novels I've ever read. To me, they mainly suffer from a) Streeeeeettttcccccchhhhhhhing to fill a trilogy and b) an authour who clearly wasn't familar with the BT universe.
For me, the worst of the series is Patriot's Stand by a long, long way. It's still not the worst BT novel ever (o hai thar Star Lord, how's it going?) but it's pretty damn bad at the best of times. Paper-thin chracterisation, moustache-twirlingly evil villains who are as dumb as a bag of rocks, sickeningly pure and nice and shallow protaginists and again, a writer who doesn't know the BTU from his elbow.