I totally agree. I think the Reaver is right where it needs to be. The problem I see is the disparity in difficulty,
The larger problem is the fact that Reaver is concerned with damage thresholds.
Let's assume we have a Reaver who is played fairly poorly -- the only thing the player knows how to do is this cycle:
Dragon Rage
Dragon Rage
Dragon Rage
Devour
Repeat infinitely. The question now becomes "Will the Reaver take enough damage between Devours to actually kill the Reaver?" Well, the amount of damage that would require is what, slightly over half her HP? If she takes less than that amount, she will never ever die and can fight indefinitely. If she takes more than that amount then she's toast.
Can think of it this way if you'd like: imagine a character that regenerates 1/6 of their HP per second. They'd be gods in Routine. Even in Threatening it'd be hard to kill them. In Perilous you start getting worried in certain cases (hi Fire Wraiths/Desire Demons/etc). And in Nightmare you can very, very easily faceplant.
Perhaps it'd be better if the Reaver's healing scaled differently depending on difficulty (meaning say, less on difficulties below Perilous and possibly slightly more on NM or Heartbreaker, but the important part is "less below Perilous") -- you still have your Heal on Kill/Heal on Hit stuff, improved armor, and bonuses from Promotions to help.
If Devour/Rampage healed for 40% of their current value on Routine and 70% of their current value on Threatening, for example, I think the Reaver would still be quite fine. She doesn't *need* to heal as much because she's taking less damage in the first place.
That said, I'm not even sure it's worth changing...but it's the incoming damage vs healing done ratio that's the problem (at least for people without Dragon/Hakkon weapons and 30% HoK or something) in terms of her survivability, at least.