the maxim of rolling: There is nothing easier than rolling a kayak--with good technique.
There is nothing harder to do than to roll a kayak--with bad technique. No amount of strength can overcome bad technique.

Jay covers the important points of design.
To that I'd like to add: good coaching, practice until you can do it without thinking, and have someone photograph you or videotape you and look at it, maybe with that good coach. One photo of me rolling showed that I was sitting up too soon. Before that I couldn't figure out why I was missing some rolls.

I'll bet you could put a great roller in the worst kayak and a bad roller in the perfect kayak and technique will win out.