You are very exposed out on the end of two lines in fast moving water.  You are relying heavily on your belay team which can't hear you.  Setting safety below is always a good idea.  What if something goes wrong and your quick release is stuck?  Sounds farfetched?  Not exactly, something similar happened to a SWR instructor during a class on the Cheat Narrows a couple of years ago.  He was on a zip line and his quick release belt failed to work correctly.  If breathing OK, perhaps your belay team can get you quickly to shore.  If not, this is where your rescue knife comes into play.  You may need to cut the belt or tether.  This is just one reason we always carry a rescue knife when working around ropes.

V-Lowers take time to set up properly.  Always use a rescue vest so you can make a fast get-away.  In heavy turbulent water, the rescuer may porpoise and get pulled under water periodically.  The belayers and rescuer work as a single team, good communication via hand signals is vital.  V-Lowers with a raft are usually a bit safer if set-up correctly.  Overweight the downstream end of the raft so it doesn't take on water.  A quick release knot is an important part of the set-up.  As with most rescues, set appropriate downstream safety.

The main advantages of this technique is the ability to position a raft or person at the accident site with pinpoint accuracy.  If lowering a raft into position, this typically provides a very stable rescue platform to work from.  Some of the main drawbacks are it takes a fair amount of time to set-up and setup is often very complex.  It is pretty easy to lower persons but very challenging to move back upstream.  Use of a single wrap around a tree is often necessary as the forces are far higher than most people expect.  Many fail to set a deep enough angle, a 45^o angle or less is ideal - basically a very deep V.

Picture a rescue scenario where the victim is perched precariously above a high ledge or falls.  Do you really want to take a chance going over the falls if you slip - probably not.  This is what v-lowers are absolutely great for.  Your teammates place you where you need to be with pinpoint accuracy and safely.  Since you are very secure, you can work with both hands to free the victim and securely grab them.  Lots of hands on shore can safely retrieve the two of you.  Two independent lines also provides an extra margin of safety.