Stan in general is about moderate level on surfaces outside of clay.
He can produce on slower hard courts, not really the faster ones (IW and Miami last year he was excellent). As you say, all about his backswing mechanics.
However, he has made the 4th R at Wimbledon at least twice, so he can play decently on grass.
Yeah, but check who he played. Vassallo/Schwank/Levine last year - Levine is competent at times, but I could beat the other two on grass :-) 2008 it was Daniel, Potro, and Zverev by retiral. His game worked against Murray because he wasn't pressured, in very heavy indoor conditions after the ventilation systems failed. He could set up and tee off big hit after big hit, while Murray couldn't do any damage at all.
Against a heavy serve and powerful groundstrokes he's toast - I saw him Nottingham one year, he was even overpowered by the mighty Seppi groundies, could barely win a point against that giant serve. Sure, Notts is a lot quicker grass, but it's the same general problems Wawrinka has on any faster court.