This has been discussed on the TR podcast a few times I think. They tend to suggest the following IIRC:
1) Be sure that your FTP is set right.
2) If you’re ill or too fatigued then it might be better to just call it off or opt for a lower intensity workout instead, but if you’re good on those fronts then:
(a) Dropping the intensity a few % is OK. Different people do have slightly different power profiles, so whilst e.g. 120% of FTP for n minutes might be a do-able VO2max interval for many, it might not be for you, especially if you’re a bit old or weak of mind (:-)). Drop it by too much though and you will miss the point of the session.
(b) A super-short rest mid-interval is OK. They tend to recommend back-pedalling for a few (<5) seconds just to give the muscles a chance for a quick flush and the mind a quick break. The way they talk, they seem to do that themselves often – much better to do that and still get through the interval than to bail or drop it down.
(c) If the high intensity stuff is still regularly too tough for you to complete, but you’re sure that overall your FTP setting is OK, then you can increase the rest between intervals a bit.
(d) MTFU