I wouldn't necessarily agree with that - I have just this, an El Son piezo tweeter (apparently as used in some Trace p.a. cabs) and it works fine for me, just filling a bit of top end where the Barefaced Midget runs out. It isn't overly hissy either, it adds a nice click to string on fret if you like that sort of thing, but for me it just makes the fretless sound a little more acoustic, or natural-sounding, for want of a better expression.
It has some built-in crossover (no idea what frequency, but probably quite high) and no attenuator,but the treble control on the amp controls it's range fine.
Maybe I just got lucky!