There are many abilities that the mobs can have, and they are as follows:
1UP - Needs more testing, but it appears to basically cause you to have to kill the mob twice in a row (hp fills up once it is reduced to 0)
Berserk - Appears to be a damage increaser.
Blastoff - Can launch the player into the air (even from a distance).
Bulwark - Significantly increased defense.
Darkness - Causes a blindness effect.
Ender - Allows the mob to teleport like an Enderman.
Exhaust - Needs more testing. Could be that it consumes a players hunger faster.
Fiery - Can ignite the player.
Ghastly - Needs more testing.
Gravity - Needs more testing. May limit the players ability to jump?
Lifesteal - Steals the players HP to heal itself.
Ninja - Can teleport after the player.
Poisonous - Can poison the player.
Quicksand - Applies a slowing effect.
Regen - Allows the mobs health to regenerate.
Rust - Causes the players armor to lose durability faster. (Possibly weapon too).
Sprint - Increases movement speed.
Sticky - Causes the player to throw their weapon on the ground. Disabled in the mod pack by default.
Storm - Needs testing. May hit the player with lightning?
Vengeance - Player hits themself for 50% of the damage they deal to the mob.
Weakness - Applies the weakness debuff to the player.
Webber - Places web blocks where the player is standing to greatly reduce mobility.
The Epic Mod Pack has a specific loot table for infernal mobs, however I do not plan on releasing a complete list at this time. The drops span from cosmetic blocks, to ores, to weapons and armor, to some of the highest teir items in the pack. There are many items that may only be obtained through killing infernal mobs, and they are always worth attempting to take out.