The piezo on my Markbass CMD 121P absolutely drives (pun intended) me up a wall
Peizo's are weird, man. For one, they go up into dog whistle range but the response is anything but smooth.
The odd thing is, if one sounds bad, more can sound better. Makes ya go, ???
Multiples help smooth the response and split up the power so they're not overdriven into harshness.
I use 2 along with a single Beta 10" in little vocal wedges I made. They're not hifi or boutique by any stretch of the imagination, but they get the job done and don't give you ear fatigue after a 4-hour gig, mostly because we didn't have to turn the amp up far to reach the volume needed so didn't drive anything into harsh tone.
I made a 210 bass cab once I put 4 of them in. That would get bright and snappy, but again, split up the power enough to not get driven into "harsh". Either that or they were acting like enough of an array to direct their sound under my ears instead of into them.
Strange creatures those little things are, and definitely not for everybody.
BFM has some filter schemes to steepen their natural rolloff and filter out the harsh but hardly anybody uses them. They just string them along with whatever other speakers are there and let them do what they do.