They are not so useful on fretted basses, as they accentuate a lot of the clank, finger noise, and treble that don't work well in most settings.
The systems using transducers in a typical metal bridge seem especially prone to causing more problems with technique.
The Rob Allen-Turner Renaissance style of using an undersaddle transducer in an acoustic style wooden bridge on a fretless are where the magic lies. Very versatile (except for slappers!) and useful in a lot of acoustic, jazz, pop music. Useable in roots and rock as well. Deep, rich, full of beautiful detail, and easy to use and make sound good!
Most bass players won't even try out out the Allen-Turner style instruments, figuring "they are not for me" based largely on their appearance. So not popular in the mainstream.
They just don't know!
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