Pu adhesive sticks well, but you need to hold whatever you are sticking firmly in place whilst it sets because it expands as it cures, though not as much as the expanding PU foam. When using it for wood work you have to clamp tight because it expands and forces itself into the grain of the wood.
Pink grip is, granted, far less flexible than marineflex, which is after all a rubber type sealant, but it is not inflexible. It is in fact more flexible than the steel to which it attaches. Before using Pink grip to stick celotex to steel I have used it to fix battening to my steel narrowboat - that was ten years ago. I tried to remove some of it recently. I had to resort to a bolster chisel and lumphammer, and even then the wood sheared off leaving a thin layer still glued to the steel. Pink grip is crap if used outside as it doesn't like being permanently wet, but inside, it's great.
But this is just my own experience, you must do as you see fit.