I am no longer on the SOR so don't have any issues but used Eurostar extensively when I was.
I never had any questions at St Pancras on the way out. The French border control didn't seem very interested.
On the way back I did have an issue at Brussels where I was lead away with my wife into a holding room where I was asked some questions and the border guard took our passports to be photocopied. It was deeply unpleasant and would obviously be very hard to explain away but he did not reveal any details about why we were stopped. This occurred not long after the new SIS system came into play with exit checks so it could have just been them getting used to the new system.