I think you’re getting a pretty good understanding of the limitations of the ramp test. Overall, it works pretty well for a lot of riders, but there are exceptions as you pointed out. There has been a lot of discussion about this in other threads where people have pointed out that it doesn’t work well for them for one reason or another (myself included). As a result, most of us just adjust their FTP manually using the ramp test as a starting point, or adjust the workout intensity for a given workout. It’s going to take some experience of doing the ramp test and then seeing how your training efforts actually feel at that level (and then tweak things as needed). The way I think about it, it’s not really trying to determine your “actual FTP” accurately. It’s really just trying to “calibrate your training plan efforts” based on where you are at the time. When I first started with TR, I think I got too fixated on the number. It helps to understand what are your training goals?
As an example, I am a 61 year old endurance rider and my point of failure on the ramp test is always due to hitting max HR (175) and breathing, so I am limited by my VO2Max and not my legs. The number I get from the ramp test always seemed too low to me because using that number I found that the SS and threshold efforts felt a bit too easy. On the other hand, on those few VO2Max workouts in my plan, the ramp test FTP number feels about right. I can complete them, but I’m about to fall off the bike at the end. Feels very much like the point of failure on the ramp test. I think this is telling me that I need to work more on my VO2Max. But then, I don’t race and don’t ever intend to be in a bunch sprint at the finish line, so I don’t worry too much about it. I do want to be a stronger endurance rider and be a better climber though, so that’s my focus.