Your question is very, very complex. Well, not the question as such, but the things that have an effect to the situation you describe.
Anything that happens suddenly in an airplane has varying effects, depending on at least (but not limited to)
- has the pilot received proper training for such situations
- is the pilot proficient with regards to the situation (I don't mean in a regulatory sense)
- what is the personality of the pilot like
- what is the psychophysiological state of the pilot
- in case of a multi-crew flight, how well is the crew coordinated
If any of the aforementioned is less than satisfactory, it is reasonable to assume something might hit the fan. And talking about reasonable, if something actually goes wrong, one way to describe the failure is to use the "Swiss Cheese Model", aka "the Reason Model" (not because it deals with reason or reasonable stuff, but because it was created by D. Orlandella and J. T. Reason).
Now, the IR rating itself, while of course is necessary for successfull flight in IMC, does not at all guarantee survival in poor visibility. More important is experience, and self discipline. You see, I'd say that marginal VFR is harder to fly than dead zero visibility IMC. In IMC there is usually very little to distract you, and since you have nothing to reference your situation with, you stick with the gauges the best you can. If there is some visibility, the risk is, that an inexperienced pilot, or one with poor airmanship discards the gauges and loses situational awareness.
Professional pilots that fly alot have probably the best abilities when it comes to situations like the one in question, if company culture is of the right sort (strictly safety oriented).
Flying by gauges is not that difficult if training and practise are in order. Perhaps the most difficult part is to recognize when to switch from visual to instruments. There absolutely should be no in-between!
To add up, I have to say that a pilot that finds oneself suddenly and unexpectedly in IMC is not a very good pilot. We all make mistakes, that's for sure, but if you are properly trained, have been practising rigorously, are physically and mentally fit and not the sort of person who either freezes or acts overly confident in surprizing situations, it would be weird if anything went horribly wrong.