No drawback, the manga's nearing its conclusion so Luffy being this strong doesn't break anything, Oda can easily write Imu and/or Blackbeard to be able to counter it in some way.
It's definitely a sad day for Luffy vs Akainu agenda pushers though because Akainu's got zero counter to it and gets Blueno'd
but then what’s the point of his other gears? i was kinda hoping gear 5’s drawback would be something like: “he can never use gear 5 again”; “his devil fruit abilities are taken away”; “gear 5’s drawback is a combo of every other gear drawback”; or (my favorite one) “he deflates into looking like a really weird c0ndom (they ARE called ‘rubbers’)”; or a combination of any/all of the stuff i mentioned… and what’s the point of calling it a gear if it isn’t something luffy can and HAS TO (not ‘can and WANT TO’) switch in and out of? like luffy gets tired after gear 2, gear 3 reduces his mobility, (although, previously gear 2 would damage luffy, while gear 3 would shrink luffy) gear 4 takes away his haki, and pretty much all of them make him hungry
tl;dr - a want gear 5’s drawback to look like a c0ndom, because that would be really funny