Oh boy. Well, it depends on your perspective. If you want a healing staff for hardcore PVE content (such as Dungeons and Trials) this is one of the strongest if not the strongest and there's a reason most teams have one healer running it at all times.

First, you can wear this with a monster set and two 5-piece sets, and you can easily frontbar it. There are a few frontbarring options for healers but this also comes with the strongest perfected bonus for healers ( +~900 Magicka).
Then, the active effect grants ~470 Stamina AND Magicka to every ally hit by it.

This is adds up over a fight, and it is a considerable amount of bonus sustain. It's almost like getting an entire extra champion point star (as it's very similar to Enlivening Overflow).
As well, each ally it gains value, making it exceptionally efficient for 12-person trials.

So being able to add it without breaking your other sets, giving the most valuable stat possible for healing, and providing a nice sustain buff to the team, it becomes clear why this staff has been meta for so long and there are few other options that are actually run in hardcore content.

In my opinion it's the best choice for 95% of the pve raid bosses. Why would you want to buff it? That's almost like asking for a buff for maelstrom inferno staff.

It's an 8m radius, so in most fights you can manage to stay inside most of the time. And if you have to move further away (e.g. after the boss jumps in CR) then you want to recast your grand healing anyway.
Also many healer sets can easiely played only on your backbar, so I don't see this as an issue (Roaring Opportunist, Powerful Assault, Martial Knowledge, Saxhleel, Stonetalker, ...).

However I agree that some of the other healing options would deserve a rework. I think I never even considered using timeless blessing (AS healing staff) outside of pvp and force overflow (vateshran) looks good on paper, but when I tested it it worked quite badly, probably due to some boss hitbox sizes.