example: true namer without these feats goes on a multiplanar adventure that takes them to baator at some point. Does a bit of offensive truename magic a few times in front of some minor devils. One's an imp who used to be an elven wizard or something. Knows a liiitle bit about true namers (very likely) IMMEDIATELY flies like the wind invisibly to the greater devil (who knows a good deal about true name magic as many greater devils do both because its useful for writing infernal contracts and because greater devils fear true name magic in the hands of mortals as its used against them and is part of why they learn a little about it as self defense against thise who have it as awareness is the best defense) who is immediately above his own master in a bid to get out from his current masters thumb (very likely) and tells him exactly where you are whis with you and any other info he can as well as any non consual use of a true name he thinks you used which he percieves to be an infraction of any sort of infernal law that you may have broken. You have spent a few days in baator and now youve already done goofed enough to catch the attention of a greater devil probably way over your party cr limitations. You will probably be met by a couple pitfiends in a couple hours armed with at least one wand of silence just for good measure. Truenamers and multiplanar campaigns dont mix. Now you have a great example of why.
Was thinking about this as well. But in different terms.
First of all, there is nothing in the PHB or MM, that would give you direct control over, for example a fiend.
In the case of the devils you'd need their name on an amulet to gain full control. The name is not the problem, but the sacrifice of (typically) somebody the PC loves isn't something that'll occur too often. Either by player's choice or by DM's intervention.
Demon Binding too requires blood of the innocent etc. etc. otherwise there is nothing to be found except that you gain some measure of control over the fiend, like in the case with devils.
Now what is that measure of control you get?
Say you cast Summon Greater Demon.
That demon get's disadvantage for its saving throw against your summoning spell if you get its name, that's it so far. It still gets to make a save at the end of each of its rounds, if it wants to.
Arguably in the case of this spell, you could just order it to tell you its name. No onomancy required. Would fit thematically though.
The same goes with Infernal Calling.
There's not too much you can do except give them disadvantage against a saving throw at this point. The whole shtick with the power of names and gaining control seems not as threatening as depicted in the MM when expressed in mechanical terms.
Also a thing about wording. You "mentally learn" the name of the creature you're using Extract Names on.
As long as you don't intend to keep that creature alive after either casting Bane upon it or use a Resonant Utterance, it won't matter. If you intend to keep it alive you can either choose to not speak its name until a point where you need the name. It won't know.
Afaik it's unclear whether any kind of creatures have knowledge of their true name. They might know OF true names, but not their true name. At least with humanoids on the material plane that seems to be the case. Wouldn't rule that differently for fiends.
So even by speaking a fiend's true name, it could be argued that it doesn't know that it name has been spoken (as part of the bane spell for example).
Now in your example of Baator. I guess a devil would think twice about getting the news spread around that there's an onomancer on its layer. How would that devil know, other than having it's name extracted? That imp/devil will not have it's name changed upon promotion.
You could turn your argument around and say devils have way too much to lose if somebody knows their name and will likely be very careful whenn approaching that problem. And the onomancer is a valuable resource to many a devil.
There is also no official ruling about how much devils know from their past life. Lemures don't remember squat. That's in the book. So as long as you didn't get your infernal rank through selling your soul, that's not a real danger. The elven wizard who very likely knows about onomancy but is now an imp example is a bit hypothetical.
I'd say as long as you play it smart, which as a wizard, you probably should be. It's fine.