Came here wondering the same thing.

same

i did some very minimal testing: damage dealt to a dungeon end boss without the legendary effect and with it.
this is on a barbarian, using bash (the legendary effect was on chest armor, only basic abilities doesn't get amplified by chest armor benefits)
lvl 73
chest effect deals 47% more damage to unstoppable enemies

with legendary effect active (having the chest armor equipped), each swing dealt about 2.6k damage +/- 100 or so
without the legendary effect (chest armor removed) each swing dealt about 2.6k damage +/- 100 or so.

i put it on and off to make sure for irregularities

at least for end-dungeon bosses, which feature the stagger bar mechanic that rules how CC is applied to them (one would think is "unstoppable"), they do not qualify as "unstoppable" as the effect of the legendary does not increase damage against them

I did the reply with the minimal data test. Still not logged in. Whoops. Editing came out like **** on that previous explanation, but long story short: from testing, end-of-dungeon bosses, which use the stagger mechanic, do not count for "unstoppable" because the legendary effect that amplifies damage to unstoppable enemies, does not amplify damage at all to them. Bummer!

This please!

bloodmist gives unstoppable, but it doesn't say so in the tooltip