You can now Mana Burn and Devour impervious troops, though the trait still says that troops with impervious should be immune to these effects.
Its well documented that Impervious is broken. I was not aware of Mana Drain having effect though but the devourability of Impervious troops is well reported.
It’s in the OP of the known issues thread.
And I know I posted somewhere about mana drain.
I have proof of Eleumaugrim getting mana burned.
It happens at 20m:50 seconds.