I've had two coaches with galley slides, for a total of 8 years. We did have one plumbing problem - the manufacturer did not leave enough slack in one sink drain line and it eventually broke where it joined a fixed pipe. I've heard of a few such problems in other rigs too, but the number of rigs with galley slides has to be in the many-thousands.
Obviously, moving plumbing, electric and gas has more risk of failure than stationary equivalents, but all in all it does not seem to be a significant issue. I bought a second one after my experience with the first (including the problem), so you see how I voted on the issue. I would not let the slight risk stand in the way of the obvious benefits.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL