Zubr, let's discuss this.
Why can't he use the "Assassinate" ability if he has immunity?
No, what do you mean he CAN'T? He can, of course. But he might also reveal himself. And he needs to save his mana. Besides, he's safe and can use "Scout" to then effectively use his ultimate ability. At least, that's what I would do, maybe.
И пока в кармане пачка милкиуэйев - значит все не так уж плохо на сегодняшний день