I understand the point your making here, but my point about the slime transformation being the only Self transformation menu outside of buggy that still has full intended functionality still stands.Slime TF is the only one that's meant to be that good at this point in time.
The most recent horn bug aside, Demon TF is supposed to start off in the current highly-limited state - it only gains more functions through main quest advancement.
The player character gets to add human TFs to their free-transformation repertoire due to the perk Lyssieth gives you, giving their demon form TF slightly better capabilities than those of generic demons off the street.
Presumably future Lilin encounters, once defeated, will grant a similar perk that adds their racial TFs into your PC's demon TF.
Eventually it should be superior to slime TF (since it will have all the allowable types but not lock you to being "slime-textured" with fathomless orifices that are always drooling wet), but that will take years at the current pace of MQ progress.
For now going demon is only worth it to see the hot one-off demon-incest orgy scenes, and unlock a few options during quests where you can intimidate others by openly parading around as a demon (or, if you're disguised as a human-demon, reveal yourself before exerting your intimidation).
In terms of freedom of transformations, going demon is worse than using elixirs (or temporarily going slime) in the current state of MQ progress. Since money and essence is not a gate at all, "free transformation" isn't really that big of a draw when it is so restrictive, and demon is a permanent template (unlike slime).
Edit: at this point, The transformations are the least of my concerns. but for all I know, Inno will prioritize fixing them over things like the completely shagged offspring map or the excessive memory leaks.