I don’t think longer intervals are necessarily better, because (1) the element of pacing becomes very important and (2) you need to have a good ballpark estimate for your FTP. And (3) you still have the same issue that you have with the MAP test: you need to convert your 20-minute power to your FTP. Coggan initially knocked off 5 %, but for the general public that seems not enough (on average), so nowadays the recommendation is 15 % — unless you know what you are doing. Perhaps the statistical distribution is a tad narrower, but you still have a range of (at least) 10 %, which is a sizable margin of uncertainty, big enough to be significant IMHO.
Now if you are experienced, none of this is likely an issue, but then most could also use a MAP test to gauge your FTP (corrected by hand with experience to account for your personal relation between MAP and FTP).
That’s not universally correct, the easiest example I can think of are over/unders. They only target the correct energy and lactate systems if the over intervals are really over and the under intervals are really below FTP. Here, I think you really need to be in the right ballpark within 3-5 % max. Otherwise 105 % FTP-as-set (a common value for over intervals) might be <100 % of your actual FTP. Or you need to go for 110 % FTP-as-set, which could make these over/unders prohibitively hard.
The better solution is to pick your favorite FTP test, and, importantly, to not be afraid to correct it downwards. I find the ramp test better, because it is an all-out effort. That is, done correctly, you’ll be hugging your bars after it is over and panting. Yes, fixing your FTP as 75 % of your MAP is problematic, but just use common sense and double check. If necessary, tweak it. IMHO one psychological issue is that we (I’m including myself here) are much more reluctant to correct our FTP downwards rather than upwards. Once you get experienced, I find you can guess your FTP-as-tested within a few watts. That is, you develop a feel for how strong you are and how hard a particular wattage feels, which are obviously extremely valuable skills.
Like you wrote, you can verify and should whether the FTP you have set is reasonable by doing a few choice workouts or do outdoor rides. I usually like to use a threshold and a VO2max workout. On outdoor rides a good rule of thumb is that your 20-minute power on an outdoor ride is larger than your FTP, your FTP is set too low. That is assuming that you have e. g. an uninterrupted climb that is at least 20 minutes long. Or you can also use your 5-minute power PR, that should be roughly 125–135 % of your FTP (unless you really just do an all-out KOM attack-style effort).