I can see no real disadvantage for using a loop except it shortens the draw length of your bow by 1/2"-1" depending on your loop size (so you will loose some speed due to the shorter power stroke).
If you choose any number of "Loop specific" releases on the market right now with short heads and forward trigger designs you don't have to give up ANYTHING to keep the same anchor point you would have shooting a caliper off the string.
Believe it or not in some cases with a properly sized loop, and certain loop specific releases it is actually possible to INCREASE your draw length a bit depending upon what release you are switching from.
That old addage of having to shorten your draw length to compensate for the added loop length doesn't need to apply any more if you gear your release to the loop and not just try to adapt your old one to the new style.