I have not seen a meaningful difference between indoor and outdoor FTP, even with using my trainer as the power meter indoors and Assiomas outside. I don’t do FTP tests outdoors, but do long and hard enough efforts to have a good idea.
I think where a lot of the difference comes in is cooling/thermal issues. I think that many people underestimate the amount of airflow they need indoors, and this can be a real limiting factor. If you have poor cooling indoors and this limits your efforts, then I think you will see a lowered training effect, similar to if you only rode outside on really hot days and the heat limited your power output. If the ‘core’ body temp sensor goes mainstream (and is as accurate as claimed) we may be able to get more data on how much of an effect thermal issues are indoors.
For some efforts, I think that indoors will likely produce a better training effect, as it is much easier to have constant work for long periods of time. 2x45min at sweetspot? How many people can do those efforts outside without some breaks? Indoors there is nothing in the way. Obviously the amount of breaks matter, but I would be hard pressed to do those locally - I either have stop signs/lights, or non-trivial downhill sections. I doubt there is good data on how a few 30 second breaks in a 45 minute interval effect a training session, so things like that may not make a meaningful difference even if indoors seems ‘better’. I also have a hard time keeping a consistent ‘easy but not too easy’ pace outside, and I think the trainer offers a good option from a training stimulus there as well, but it is not as enjoyable as riding outside.