For a newer class C, it's very nearly a moot point; you will get one one or more slides with the great majority of motorhomes available. There are not too many non-slide floorplans available, and those that are I suspect exist mainly to satisfy the rental market.
Slides are not perfect and do occasionally fail, no doubt, but for the most part they must be pretty reliable or they wouldn't be so nearly ubiquitous. Plenty of other things in RVs can give bother now and again, too; generators and furnaces and air conditioners and roofs and doors also need maintenance and the occasional odd repair to work properly and reliably.