Piezo pickups are definitely useful, but they have one really serious problem: Impedance matching. Having an impedance mismatch is like having your car in the wrong gear, and piezos really need to see a very high input impedance to sound good. If a piezo has to drive a low impedance the sound will lose a lot of low end and also sound bad in the mids and highs. There are a variety of good “acoustic” preamps available to help with this, including built-in electronics and floor units. Trouble is, they’re expensive and usually include EQ and effects you may not need. Not all piezo pickups need this kind of buffer, but I’ve run across a decent number that do.
Often enough I’ve done sound for musicians with just this problem, so I finally ended up building a few very simple, very cheap buffer boxes that I could carry in my bag and even sell cheaply if a musician wanted to have one. It runs off a 9 volt battery, uses a single FET (field effect transistor) and a few other parts, and can be mounted inside one of those little tin mint boxes.