OTSTBs are very popular here…actually, there may be a whole bunch hitting the market shortly (Alberta) as one of the major tracks just closed.
I rode a former pacer on trails once. The owners had decided to give up on getting the horse to trot an canter and just enjoy her gaits the way they were
It was a fun ride! Sweet horse, not afraid of anything, and lots of get-up-and go.
I have met a few others re-trained under saddle. It usually didn’t take much to get them from ground-driving to under-saddle…generally very trainable with sweet dispositions.
Of course it is a horse-by-horse thing, but in this area there are probably as many OTSTBs as OTTBs…and I hear more good about OTSTBs than bad. Often you get surprised at a show to learn that a horse you thought was a WB or Sporthorse is an off-track standardbred!
That’s all from a riding perspective, though, hopefully that helps a little.