In Legends/Canon occasionally a character will enter a period of reflection, and will come out stronger or muse on his/her growth in the force. When they embark on their inner journey, they may come out more powerful or compare their current standing to how they were in the past. Beings like Darth Thanaton and Vader have had inner battles with prior incarnations and mused they were more powerful, but how reliable are these determinations by force users to gauge their growth in the force? Wouldn't a force user whose learned more techniques, had more growth through experiences, and gain more training become more powerful or are they at best the opinion of the force users and nothing more?
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
I think they should be reliable in most cases. They're the ones who know their power best, so their inner-musings about it should be relatively accurate.
It's not really reliable at all. As you already mentioned, in some cases the portrayal is heavily dependent on character perspective, in others it's being influenced by a third party to produce a specific result, and sometimes the sequences are entirely metaphorical too.