We use wavy seas on many of our custom installs and have yet to this date had a faulty unit.
If used properly, a wavy seas will not reduce the amount of flow in a tank.
It will however, help to distribute the flow coming from a return a pump or bring much needed water flow to "dead" spots in a reef.
Wavy Seas are very versatile in that they can be paired with a Tunze, MJ, or main return from sump in order to help get water flow to those hard to reach places.
In many instances we have used them as the source of main flow in a tank when coupled with a large return pump such as a Reeflo Wahoo or similar. These are great units if you do want to add ugly powerheads to your display and want to keep everything hidden.
I agree with ZZROCOOL. Depending on the shape of your reef, you may need several different devices to achieve the proper flow where you need it.
It is of my experience thus far that there is no singular device that will get water all over your tank. It takes a of creativity to get full coverage.