How do you use demons?

I mainly use 2 demons, and I specifically run them when I see people building glacial comps. However, my favorite way to use a 2-demon strategy is to go ranger knights and turn one of the rangers into a demon. I find this super helpful against assassin comps running Pyke because you can just deny his ult, and it's strong against Sej as it can prevent her from opening with her ult.

However, I don't really like using more than 2 demons. I just don't feel that the mana burn bonus is strong enough to justify 4 or more slots on a team.