Given that this is Meloetta that's being discussed, it's not like it has that many other options. Unaware is useful against Gengar, due to it not usually having particularly powerful coverage options, but against something like Regigigas or MM2X you'd have a better chance with something that means you can possibly survive after switching out.Because Gengar can Spore you a second time from the grave, right
And honestly, I'd rather use Poison Heal on something that is actually a threat, rather than a tech for a specific threat. PH should definitely not just be thrown around on whatever you'd like since you can only run two of any ability per team - save it for something that can use it well.
Poison Heal is ridiculously overrated. Good, yes, but overrated.
On the other hand, this is still Meloetta. You could do something sensible like just grabbing yourself a Yveltal and giving it Sucker Punch rather than wasting your time arguing it, it sucks most of the time anyway.