Now i know my opinion of demonlord being not all that bad is very unpopular but hear me out. Demonlord doesnt give the longevity of your minions like well trained does but instead compensates for if they do indeed die you can have some other minions to rely on. Its best used with a bow because the bow has range and it attacks fast the faster attacking the weapon the more demons are going to come out, daggers would probably be good with demonlord aswell given how fast they attack but i havent tested this out. If you use your typical strategy of just letting your followers do all the work then demonlord wont work for you, you have to get into the fight aswell.
Now ive tested out demon lord vs no demonlord against the giant crocodile boss using a pike for 2 of the times and a bow for the other 2, demon lord with the pike was faster than no demon lord by around 10 seconds which isnt a whole lot but 10 seconds isnt nothing. For the bow demonlord was faster by almost 20 seconds. For your information when i wasnt using demonlord i was using well trained and i was using three zombies for the test as theyre cheap and expendable. Even though demonlord was faster the zombies were significantly more damaged given they didnt have as much health as with well trained. But going back to my point earlier demonlord sacrifices the health for more numbers given you have an army of demons backing you up. And one of the reasons the bow was so much faster was because i did admittedly get pretty lucky with how many demons spawned, which is the nature of the perk you either get lucky or dont.
What i think needs to be changed about demonlord is that i think the demons should be counted as followers. They dont need to be controllable but i do think they should benefit from the other corrupted authority perks which are frenzy, fleshbond, and devour. I do believe demonlord would be tremendously more viable if they benefitted from these perks. If the demons lasted longer that would also be greatly appreciated. Also another thing, if the demons aggroed the enemy that would improve the perk aswell.
If you read all the way through this rant then thank you
TLDR demonlord can be somewhat viable if you use it correctly but it definitely needs a rework to be better