Generalizability vs. Transportability in Trials

ESMD:

Question: Is the most fundamental problem with NNT the fact that it relies on an assumption that the absolute event rate in a given RCT arm would remain constant from one trial to the next?

No, that is not a valid criticism. The RD can be computed from the OR and any baseline risk and so any particular baseline risk scenario can be combined with the trial odds ratio to compute a causally applicable RD (or its reciprocal, the NNT).