I had this on my '15 for 5 years before I sold it. Looked like new coverage wise just a little dirty. What you experienced with your black undercoating is typical of the plastic-type coatings. Mine is a wax that stays flexible..it doesn't flake and it doesn't trap moisture when applied. To maintain the warranty and protection it requires an every 2 year inspection/touch up. Cost for that is $99. I took my '15 for 2 of those inspections and watched the whole process...I felt ripped off because it wasn't until the second inspection that they actually needed to apply one short burst of the black wax because they found a spot that wore off
Other than they they would only re-apply the oil to some of the internal areas.
It's expensive but works great and looks great.