I worked at Appian within the past five years for an internship. I'm less aware of their success business side, but their software seemed to be falling apart. Multiple hundred line classes were copied and pasted around, logging and exceptions were pretty much ignored, vital parts of their stack were deep within deprecated libraries and frameworks with no way out. It did not feel like they valued quality, and a bunch of people felt flat out incompetent.
That said, they've done super well for themselves. It sort of blows away this thought of successful startups being full of the best and trying to build the best product. I'm curious what will happen to them down the line.