I don't want to say your wrong about your own experience, but I think sometimes SRS can subjectively feel very unproductive even when it's working great because of the way it constantly surfaces material at the edge of your knowledge. It doesn't often give you the satisfaction of smashing through a series of problems you can answer easily. If it did, it would be wasting your time.
I also think there is a real skill to formulating good flash cards when using SRS that I for one don't think I've yet mastered.
There seems to be a lot of research about SRS but I never see any research about what makes an effective SRS flash card. Maybe it’s different for every person and/or subject. My foreign language skills improved considerably once I stopped putting english on my cards at all and used only the target language + pictures. But, like the person you replied to, using Anki for anything other than language learning has been mostly a waste for me. I do suspect it’s more to do with the decks I’ve created than with the SRS process.
I also think there is a real skill to formulating good flash cards when using SRS that I for one don't think I've yet mastered.