I purchased a used (but seemingly in great condition) 2015 Denali four months ago. Since the purchase, I've had the driver's side interior A pillar molding pop off. Clips broke and it had to be replaced. I've had to replace the OnStar/Sirius XM shark fin antenna - popped an error code a month into ownership due to a leak and had to be replaced. Seems to be a common problem I've seen on the forums here. The applique moldings on the doors are all coming off - another common problem I've seen on here...I've put a band aid repair of double face tape on them for now....and the latest and greatest of them all is the AC compressor ceased up and needed to be replaced to the tune of $1200. All this on a vehicle with 50k miles on it.
Needless to say I am not getting the "Professional Grade" feeling right now.
2015 Suburban LTZ w/42,000 purchased new in Dec 2015 (~2.5 years in service)
May 2018 -> Sharkfin antenna water intrusion failure. GM said my vehicle wasn't covered and offered zero assistance. Dealer originally quoted $600, but agreed to do it "at cost" for $400. As you're aware, this a well documented issue for which a customer satisfaction program exists to repair at no cost.
When antenna failed, prior to dealership replacing, water flowed from gasket between antenna housing and roof when pushing slightly on antenna cover. Despite obvious water issue (I kept antenna, and it still is filled with water), GM refused to acknowledge that it is related to satisfaction campain.
July 2018 -> A/C Compressor seized and snapped belt. Internal A/C compressor failure sent metal fragments throughout system, requiring replacement of A/C Compressor, Belt, all Lines, and Condenser (everything except Evaporator). Repair quoted at GM Dealer was ~$3000. GM offered zero assistance.
If anyone is interested, please ask about all the other issues with this 2.5-old-vehicle.