One can assume that since he said "coil isolators" and not "Coil Spacers" that he means the stock isolators.
The only thing I can think of, is that the rubber is not as sturdy as a poly spacer. I stacked 3 of those once to get 1.5" and after a few off-road trips, the bottom spacer, which was much over used, sort of died and that spring didn't sit right. I replaced them with TeraFlex spacers and never looked back.
Ron