I've changed my post slightly and made another one right below.
Anyway, my view is fairly simple.
Characters get stronger either through training or through power ups. Training requires time, PU are instantaneous.
DJ, G2/G3, Asura, RS even Shusui, this for me are all instantaneous and therefore PUs.
Udon prison and Enma require time to train. I would even argue that as soon as he picked up Enma Zoro actually got weaker due to its quirk.
But that's my view. Not a very strong opinion anyway, so whatever