We've owned two coaches with the kitchen in a slide, for a total of 13 years of half-time RV travel. Had one problem - a drain line that was a bit too short and eventually cracked the pipe fitting where it attached. Not hard to fix once I figured out where the water was coming from.
One can always argue that placing the galley in a slide adds several more things that can break as they move, and that cannot be denied. However, it does not seem to be significant source of problems. Heck, the slide itself is an extra piece of machinery that can break and needs maintenance. At some point you have to decide whether the increase in convenience or space is worth the slight extra cost in future maintenance.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL