Fast and deep water, waves, etc. can make it quite a challenge to maintain a consistent air pocket for a person on a v-lower.  It can also be a real challenge for a raft lower as it has a much larger surface area to catch the water.  Adding ballast on the downstream side of the raft can help to raise the upstream side high enough to avoid taking on water.  If you no longer feel your position is safe, use your quick release system.

Strong currents create a great deal of extra load on a V-Lower.  It may be difficult to stay on the surface and avoid turning over.  The V-Lower is just another technique.  Think about your situation and choose the safest and most reliable way to access the victim.