I've never particularly liked the idea of toilet training a cats for all the reasons mentioned above.
Related story - A past employee at the animal shelter I work at adopted one of the kittens from there a few years back. I remember her going around bragging about how she was training all her cats to use the toilet rather than the litter box, showing everyone pictures etc. I thought it was weird but didn't think much of it. Come to find out, about a year later, she returned the kitten she adopted because it "developed" toileting issues... Needless to say we all thought she was a moronic b**** for trying to force that on her cats then getting rid of one when it got confused.
The cat never had any issues with using the litter box once she came back to the shelter...
So yeah, there can be lots of issues with it and the poor cats can get confused. Our F1 tried using the toilet a few times as a kitten but thankfully now he's a 100% full-time litter box user. I guess I can see the appeal to some people of not having to buy litter or scoop a litter box, but imo the benefits just don't outweigh the possible risks.