This has been considered by many. The con of using a sling versus a prusik, is it limits your options. The buries make pitching the tarp close to the ground finnicky. Also, stringing the tarp in porch mode is nice, but takes a good amount of cord, which would be doubled using a sling. Means more bulk, more untangling, all when a single piece of cord and a simple hitch accomplishes the same thing.
I've heard the same - the downsides being the minimum distance allowed is greater than with something like prusiks. Another is that whoopies are a bit harder to fiddle/adjust the length when they're under a bit of tension from the tarp. Still doable, but I find prusiks to be easier.