Most of the failures attributed above to glue are actually from the open celled acoustic foam absorbing contaminants from the engine room environment like crankcase gas, exhaust leaks, hydrogen from batteries, etc. and then failing at the glue bond. In most cases the glue and a layer of foam are still adhered to the surface, the foam is what fails.
That is why manufacturers have facing on insulation, as a vapor barrier, and why they recommend taping all edges and seams.
And, acoustically the product will perform better when glued.