I have a 2014 Endorphin and an old D-T from around 2010. I know many many people who ride various Knollys as they’re a local company (Vancouver)
Prior to the 2014 Endorphin, I had a welding issue on the top tube/seat tube junction of 2012 Endorphin which, apparently, wasn’t that unusual but that’s the only frame issue I’ve ever heard of. (Bear in mind that I’m 6’4″ and 200lb and am riding their xc frame hard on the best trails that BC has to offer)
You might also want to check the Knolly forum on mtbr.
Regarding size, I ride an XL,have a long back and arms and stock stem (50mm I think) but the fit is fine for me. YMMV
Regarding “pointlessly overcomplicated”…the suspension layout is designed for best standover height, something that isn’t a problem for me but I know a very small girl who chose a Chilcotin because it was the only frame with sufficient standover height (she also used to ride a Delerium with a 24” rear wheel prior to the Chilcotin…)