I agree. If you’re training outdoors, you should test outdoors. The ramp test works for a lot of people (not me) but I’m a firm beleiver in learning how to pace a longer sustained FTP effort. I also don’t think there’s much wrong with the 2x8 version especially for mountain bikers. I go so far as to only test offroad, but I acknowledge that not everyone has that luxury of a long enough sustained climb. Yes, test on a climb if you’re MTB.
All that said, when I have a long block of indoor training on the horizon, I test indoors and expect to see a lower number. It’s just how it is, I say this specifically for mountain bike racers in particular.