I definitely see the argument against mastery that it overheals a lot. However, I truly believe it's due to not being disciplined (no kind of pun intended) as a healer / team of healers.
In a raid environment where if you don't heal it NOW then another heal will and people are meter focused - sure mastery might not be so great. In a situation where the healers work together to know what damage can be left to tick up and which healers are effective at doing so, EoL will overheal a lot less. It's obviously also dependent on fight mechanics.
Fight with predictable damage (either constant or just on a timer) EoL is very strong. Fights with random high bursts - EoL struggles if you need that person 100% NAO.
I think it's good that it's strong on some fights and not on others. Haste deals with that random burst a lot better - getting people topped up ready for the next wave. But on fights where that EoL can do it's thing you're just spending more mana to do the same healing (though if youre fighting other healers at least you'll get it first
)