TL;DR:
- FE onDeath proc shouldn't be fatal, but sometimes still does a bit of damage (seems to be equal to a regular hit from that critter).
- FE isn't passed on to minions (they used to get bonus fire damage).
- FE bosses might still have bugged melee damage (instant-kill variety).
- FE in some situations seems to have an aura-damage effect to about 6 yards that could be fatal if you linger (damage per pulse seems to be equal to regular Trash damage, but potentially hitting many times per second).
There's been a long time bug with FireEnchanted critters where their onDeath AoE and Melee hits occasionally did wildly high damage (might have been related to their max HP, since mostly Bosses were the problem).
In HU v1.7, PureRage changed the code of FE so that the onDeath proc is similar to MeteorShower rather than some form of CorpseExplosion. This seems to have fixed the onDeath AoE problem (although I still occasionally seem to take damage if standing near a dying FE critter, it's not lethal damage at all but it creeps me out).
In HU, the Barbarian Ancients have traditionally had FireEnchanted, ColdEnchanted, and LightningEnchanted mods, and the FE mod was on Korlic (the Leaping/Polearm Ancient). In HUv1.7 I swapped that mod to Madawc (AxeThrower) as a partial solution to the FE-Melee bug. Note that after PureRage's changes to FE, the Fe-Melee bug still exists, so don't engage Madawc in melee (In Hell, my cheating Sorc took around 6k-9k damage from a melee hit, so in Norm I'd expect the hits to be around 5k-7k), consider leaving him to ranged builds (consider weapon-swapping to something ranged if you're playing solo).
In recent testing, some issues were found with the new changes to FE, I'm not sure if they're actually new, or if they've always been there but everyone died before they could notice. I've only noticed this issue with Scarabs and Madawc. When within about 6 yards of the FE critter, you'll see puffs of fire damage hit you (I noticed this even up to 1 second after the critter died, so don't rush to their corpses if you're playing HC and low on life). It's not a lot of fire damage (standard Trash damage, i.e. a Sorc with 66%EShield and regular amounts of PDR/MDR (i.e. 33%TrashDmg negated) won't lose HP, but her mana would go to zero within a couple seconds). The fire-puff damage seemed to stack (FE Scarabs were occasionally quickly fatal), so I disabled FE mod from spreading to minions (this was only done to fix Scarabs, so if later Modders figure out the cause, feel free to return FE to minions).