Parson I to got taught by one of that breed.
But to be honest in those days they didn't just go straight into MEP/IR training.
They started out as Assistant FI's
Then as one of the requirements they had to have the applied instrument restriction removed which allowed them to teach IMC ratings then they did a test and became a full FI.
They could then after getting experience teach PPL and IMC do a bit of aircraft ferrying and do the MEP instructor course and teach MEP ratings. In those days there were a lot more MEP's about.
Then once they had enough experience doing that they could be fed in to teaching CPL.
After they were up to speed with that they could go on to IR single then IR MEP.
So by the time they started teaching MEP/IR they had loads of experience teaching instruments to IMC holders, they also had experience teaching MEP and just loads of experience teaching full stop. The MEP/IR was just a combination of skill sets which they already had.
The system these days just doesn't feed instructors through the relevant steps to build the experience. You are still expected to have the experience but there is no real way of getting it. And they have even managed to block people coming back to instructing. Realistically a pro pilot hasn't done VFR commercial level flying since they sat there own LST for there CPL. I will freely admit it would be an amusing flight if I had to do the test profile again without having some retraining. And that's being as current as you can get flying 6 sectors a day 5-6 days a week with pax in the back.
The IR training I think I would be OK at as its what I do as a line trainer but then again what we do in real life is different to what they want for the test.