On a frame standard diameter ST, the previous owner took off all the original paint and then powder coated it, which resulted in a thicker seat tube. When I got it I built it up with a Huret Jubilee front mech, it was a tight fit, and fatigued and broke in the clamp. Of course this couldn't have happened with a steel clamp. Other front clamp-ons are fine on the same frame, I think I have an IRD compact front derailleur on there now, to go with the Defiant 46/30 chainset. It's on a braze on to .
To be fair, I had a Campy Racing Triple clamp-on break its clamp pivot pin (a roll pin) when some sloppy shifting (mea culpa) got the chain confused as to what ring it was supposed to be on. I was running a Campy Racing T chainset in friction shifting, on that.
I cursed Campy and bought a solid steel clamp pivot pin, but somebody cautioned me, "what if that happens again and the force is taken by the seat tube?" Thanks to Ebay I replaced the mech and went on my way.