It seems like a Runic Armor type mastery which should be most beneficial to champions like Mundo, Aatrox, WW and Darius. However, the fact that it applies to ranged shields/heals makes it sound strong on champions like Sona, Lulu, Nami and Soraka. It's odd to me that it's appealing to both classes because it sounds like a balance nightmare and I also don't think many ranged supports actually want to dip into Resolve.
If Soraka can just spam W from the safety of range, that's a significant boost to her without any actual gameplay decision. If her target is low, she just benefits even more despite still being at full HP herself. Unlike when the bonus applies to your own champion being low, there's no risk for a caster support to benefit from it maximally. On the other hand, a champion like WW can consider this mastery in his thought process and choose to take some riskier fights while low HP (he takes the risk himself instead of just benefiting for free while he's max HP.
If it's strong enough to be good on supports, it will probably be too weak on melee juggernauts who want to go Resolve secondary for highly situational low HP outplays. It's really hard to allow the risk-free support benefits from this mastery (just with their normal heal/shielding spam, Locket, Redemption etc.) without toning it down too much for everyone else. The mastery tries to do too many things.
One could consider a modest melee/ranged division and whatever rebalancing is needed around that. A more fun approach is perhaps a larger boost at very low personal HP (ex: 15% at <25%). This is opposed to low shield/heal target HP so that it doesn't disproportionally benefit supports while they sit in the backline with a full HP bar. Instead of flat breakpoints, it could have a scaling. For instance, "below 50% HP, heals/shields gain additional strength, ramping up to 15% at <25%". The point is to offer a decent benefit to supports when they aren't taking any risk but providing a large benefit if they really go to save their ADC in some crazy situation where they're also risking dying themselves. Moreover, this supports the use case that melee champions want in all-ins and engages (where they actually have to entertain some risk). With the right numbers, this could make the mastery much more fun and with some actual decisionmaking rather than merely generic usefulness for certain classes.
BTW: It's misleading that the counter for Revitalize is affected even when the target is full HP. For example, if I use WW Q at full HP in a late game build, it'll increment the "benefit" from Revitalize as 8. I suppose that maybe if you get damaged during the same time that you're healing, it could actually be helpful. However, in most cases, it just grossly inflates the value in a player's mind. I'm guessing this was done for performance reasons.