It depends on the rest of the scheme options, and what kind of maps you use. Also keep in mind ZaR maps are destructable.
ZaR is fun, but imo it has always felt a bit easier than normal roper to me because of the destructable maps, and not so evil maps people use with that scheme.
Saying 15 second turn time is laughable and there's so much extra time to fool around is a bit laughable in itself lol, it mostly comes down to the maps you use.
On a lot of maps, 12s is simply impossible, even 15s some crates seem impossible.
12s is good with the right setup, 15s is good with the right setup, it's all about balance, I see no reason not to have both options.