Have you ever actually done that and got anywhere? Ive dogged a few mobs in a couple of times and got no where nothing ever happened.
When you do that remember you cant drive the car until the rwc defeats are fixed which you will be paying for them to be fixed the offending mechanic will probably be bankrupt and not have a rwc licence so they will never repay you
With the old cars I’ve bought privately (in my younger days) I usually preferred buying them without RWC (cause they were a little cheaper) and I’d always check them thoroughly myself regardless of any RWC the owner may have… But back then cars had less to no electronics so things were more basic than todays 5 star rated 4 wheeled computers
So to answer your question in a round about way, I never had any issues with having bought a car with an owner supplied RWC that also has roadworthy issues. So I’ve never had to sort out such problems.
But had I been in such a situation I’d have taken action just on principle and most definitely discussed it with the RWC provider. Regardless of the outcome the car would have needed to be fixed in any case, so either I would fixed it (at my cost), the RWC provider would fixed it (at their cost) or in an if he didn’t fix it I’d have lodged a formal complaint and in an ideal world he’d loose his RWC issuing license (and so wouldn’t be able to do his mates favours and screw over other used car buyers).