There are two camps here, that air dams provide most of the benefit of a belly pan, for a fraction of the effort, and those that say that air dams are bandages that do not solve the problem. Not many people have done A-B-A testing on it, or at least have not updated
Car MPG Efficiency Modifications Main - EcoModder. The very small amount of data listed indicates that air dams are better, but we need more data!
Who said that air dams are bad for Prii?
One point is that you should be able to tell if your air dam works, the usual issue is whether it maintains rigidity at higher speeds. My Civic is naturally low and I can still jack up my car without a problem. I do not know what other drawbacks there might be. However, with a belly pan, you would need to remove it from time to time, and if it gets damaged, it can make your car less aerodynamic than stock until you fix or remove it.