The problem is that, by the looks of it, the potency varies. If hes not EE'd, and just blasted/vaporized, hes regen is Instant, but when he is EE'd, his regen seems to take him time. We should've seen him on the show way before if he regenerated instantly, and we haven't any confirmation if he directly regenerated after It, or what he was even doing when he wasn't around.
Don't get me wrong, I LOVE DB, and DBH, of course, I want Zamasu to get full HG Regen, but to me it's seem quite iffy to apply Zamasu the full regen without much supporting evidance.