It's sufficiently niche/underpowered for standard play that I don't see anyone using it. Elemental Adept is a pretty weak feat already, but it at least has some niche appeal, for a caster who wants to focus entirely on one damage type (probably fire), and who doesn't want to have to worry about resistant creatures rendering them useless. On this one, the niche is small indeed. Characters who'd want to use this feat would have to be healers, have to have allied undead, have to care enough about those allied undead to want to be able to cast healing on them, and have to care enough about that to be willing to burn a feat for it.
Healers are a thing, sure, but most parties aren't running around with allied undead. For those that are, the standard technique is to do the necromancer thing, where you have a bunch of disposable controlled allies who walk around being bags of HP. Those aren't generally the ones that are worth spending healing magic on. Then there's the feat cost.
The problem in your case is that it's so ill-suited to a player character party that it's almost obvious that you intend it for NPCs/villains. That's especially the case because you already have a party. Presumably they're not running around with the sort of niche build that this would call for. If you're custom-building a feat for an existing campaign, if it isn't there to support some bit of significant existing fluff, and it isn't directly targeting one or more PCs, it's pretty clear that you mean it for an NPC somewhere.
My suggestion would be to try to find a way to make it something that it's at least plausible that one of your existing characters might want at some point. Beef it up enough, and in the right ways to make it worth at least considering the idea of both taking the feat and using it to heal undead. (Like, say, giving it some significant additional buff effect when healing undead.) Otherwise, it's both underpowered and overly obvious.
What will actually work for this is going to be highly dependent on the party and the world. For a generic fantasy kingdom situation, though... you're going to need to have the bulk of the feat be one thing, and the "can heal undead" be something that adds onto that. Like, say, a feat that actually makes you undead (or construct), with a bunch of benefits, that also lets your heals affect those groups. Alternately, something associated with a college, group, or ideal that gives a number of benefits, and includes "and you can heal undead" more or less as a ribbon for flavor. Finally, you might have a feat that actually gave you some sort of undead servitor that you might wish to have for other reasons, and then gave you the ability to heal them. I'm not saying that any of these would be necessarily all that easy to balance, but they might work. Anything past that would be pretty world-dependent.
Given that your world doesn't have any groups like that... perhaps consider adding one? Come up with, say, a friendly cabal of necromancers and artificers that are actively trying to normalize the use of golems and non-hostile undead in society, and workign hard on coming up with ways of making that a safe, sane thing to do. Make their ideology at least potentially sympathetic/appealing to the PCs. Add them to your gameworld. Introduce a few non-horrible spells that they might have come up with. Figure out a feat based around the benefits of their teachings, and throw "and you can heal undead/constructs" in there as a side benefit. Possibly even have them offer a quest or two to the party, with the reward being a custom-built undead servitor. Heck - the BBEG is an undead healer who clearly works within civilization and cares at least a little about her own place in society. She wouldn't want to have anythign to do with them openly lest people suspect, but perhaps she's quietly supporting them?