The phrase "suitable for the purpose" comes to mind, maybe 110.3 A (1), examined for suitability, or something like that. It's kind of a boilerplate grey area language, but probably can be found many times in the code with similar wording. In Ct interpretations come from the State and enforcement from the Town, so they do have a say about it, along with the owner (the guy paying the bill along with his agents and reps).
EMT has a lot of flex to it within a range, but vibration transmission is also a factor (or something that can be considered, when 'examining for suitability'). I like to think about it the way I think about thermal insulation. Not all R values are the same. So as you layer different types insulation, each peforms in a different way and the combination can perform much better than the same type all the way through.
Vibration and frequency transmission would work the same way. Different materials will filter or transmit differently, and a layered combination will have much better performance filtering over a wider range.
So factors to consider for suitability, equipment that has vibration, carried vibration from elsewhere that you want to stop, allowances for expansion / contraction, settling, ...
I always see flex at transformers and usually think all EMT and LB's would have been fine. The trannys have thin flex pads and bolts that you are supposed to loosen at the flex pads. But then if there's no flex and the conduit runs over someone's office, and he / she can hear it humming ... (is it suitable ...).
