We have a giant full wall slide (~32 ft) on a 3 rail system with 2 motors underfloor by Power Gear (Power Gear has been bought out by LCI). I have far more confidence in that slide operating trouble free than I do the smaller kitchen slide on the other side of the coach that has the in-wall Schwintek system.
As stated, Some of the manufacturers went to Schwintek and got a little carried away as to the size of slide they used them on, including mine, but I wouldn't hesitate to get one with the underfloor mechanisms. The full wall slides make for a nice open floor plan.
I'm on the roof all the time, and I've never even noticed the difference over the slide. I've never had any concerns for the structure of the coach.