R10 used to be group-only and was very hard for groups. It's much more matter-of-course now for a decent group and some can solo it regularly.
IMHO, reaper shouldn't have so many special negations, like the conceal/incorp cut or the R7 magic cliff or the self-heal penalty. ALL abilities should work with the difficulty scaled via mitigation, eHP, and incoming damage rate.
Each new negation is a cheap temporary difficulty that cancels a few current builds and forces the population into an ever narrowing slice of functional play as the player base adapts.
On a slightly related note: where's the background lore for reapers in game? There are ZERO quests or background on them and it's a major overarching feature of the game that cuts across every single expansion.
The game would be vastly improved with some kind of story about them. What are they doing? Why are we fighting them? Also, make lost souls only drop from reapers. It gives them an implicit story (they're collecting the souls and you "interrupted" that by killing).
Would be fun if there were "extra fat" reapers who want to escape with the souls they collected, too -- vastly more interesting if some tried to run away instead of the dumb kamikaze AI. Oh, and they agro the zone AND scale things up when they run off. Better kill it quick!
Get rid of red-alert debuffs. The swarm should be danger enough. If it's not, scale things up. I do absolutely LOVE the heart-beat threat sound-fx of it, though -- keeping that part would be nice.