The more I play around with different configurations at endgame, the more I come back to the same inescapable conclusion...
The lack of mobility and/or stationary defense is just too damning, because of how wholly dependent you are on AF stacks. It puts a hard cap on the tree, at least for any application where you cant expect someone else to be taking aggro for you. It does great single-target damage, but in any situation where you cant manage aggro enough to almost never get hit, you cant stay still long enough to make the tree work.
Its the glassiest glass cannon I've played in a while.
I messed around with a few more defensive setups: 18 Pal/2 Rgr, 18 Wiz/2 Rgr. Paladin is usually a pretty reliable tank. But even in EE, it got swarmed and killed pretty fast in Grimm. R1 was just a spawncamp in the first room. The wiz build fared much better: I could stand there and facetank on EE on the strength of DA. But on R1, that strategy failed and, again, quickly nuked. Its pretty much limited entirely by your heal rate, which makes it pretty unsuitable for Reaper even just one skull.
Against packs, it just kills too slowly compared to how fast they kill you. You feel the lack of helpful targeting mechanics when you're trying to pick the champs out of a swarm. And the minute you try to move and avoid the swarm, you break your DPS stacks and pretty much doom yourself to desperate kiting with horrible damage. Any quest with "huge swarm of mobs" design (Grimm, Slavers, etc.) is going to be effectively unplayable to a HW without a dedicated tank.
I think the only really endgame-compatible way to play it is to dump DEX entirely, which seems to be a design flaw in the tree...go INT based, 18 Wiz with Harper, and rely on your AOE CC (disco ball, mass hold, etc.) to manage aggro while you pick off mobs one by one. That's really the only way I can see to be able to mitigate incoming DPS at endgame difficulties enough that your Death Aura/NEB can allow you to facetank. Or maybe a WIS version with FvS, using Cometfall/GComm/Sburst along with active self-heals.
edit: You know what I think the tree really needs? The one piece of Inqui that doesnt really do it any good: Arresting Assault. An AOE daze would let a stationary HW archer CC a swarm if they get overwhelmed and buy enough time to hopefully thin it down without needing to retreat, since its entirely single target. It'd have to have better uptime than 10 sec/min though.