Gypsum patch used to level the concrete before a solid vinyl tile was installed (Photo 7). Proper use of a trowel is one of the problems here. Because of a lack of levelness in the subfloor, the mechanic applied adhesive to the back of some tile (Photo 8). Due to a lack of surface contact, the tile popped off the patch.
Adhesive cannot be used to level out a subfloor. There is no bridging capability built into the product. We could argue about epoxies, but your soft set adhesives are not designed to overcome irregularities in a subfloor. Should a gypsum patch even be used for this?
"What you don't see can't hurt you" is no way to address your underlayment needs. Even your low grade floorcovering will look great if the proper prep work is done the way the manufacturer intended. So if you're not sure about how to install the underlayment or if you don't know what the floorcovering manufacturer recommends for underlayment, then "YOU MAKE THE CALL" and find out.