Yes, it does get snagged under rocks sometimes and you will loose Flys. The egg sucking leech patterns that I use are unweighted for these deep slow presentations. A shallow angled down and across cast will help eliminate arc at the tip, but often where the current speed is fast enough, a slightly upstream and across presentation allows enough drift time to get the fly down deep, and the faster edges of midstream current will get the fly below the tip, eliminating some, but not all of the arc in the tip. I believe it creates a more natural presentation. The key is to go slow. Let it arc out and stop. No rush to start stripping back as the fly is now in the strike zone. Most often I just let it sit there for a bit before starting the retrieve. Leeches generally move very, very slowly. . .
Depending on the terrain, 12' of T-14 or T-10 may do the job just as well when the water is shallower and/or current speed is slower.