Inferno tactics.

at 200% it is quite difficult to make many many demons. the main problem will be building the 1st-2nd guild and buying pets.

No one is arguing that cutting down demons isn't exactly simple, especially at 200%, but if you don't learn how to do this, then you will most likely lose against serious players using this Castle. Demons are the strength of Inferno, just as skeletons are for Necropolis, only it's a paid strength :) . The Inferno respawn is usually poor, so the main task is to break through to the grass, where there is usually plenty of money, and then it's a matter of technique and skill.

when taking the Dragon Utopia and cans - there will be about 60 fewer demons.

The damage of 260 demons is such that they kill 5 black dragons in one hit even with a hero attack of 0, therefore in the Dragon Utopia none of the dragon stacks will ever hit them once, for fear of dying from the retaliation; the main thing is to bring enough meat with you to distract the dragons. In this case, all Dragon Utopias are taken absolutely without losses, and cans even more so.

suppose if looking towards Necropolis - there could already be about 1000 skeletons. and that is complete death for 200 demons.

Well, if played correctly, all the demons will remain :) . So let's consider the case of 260 horned demons against 1000 skeleton warriors:
damage dealt by demons with equal hero parameters:
260*(7-9)+20%=2184-2808 damage units or accordingly kill 364-468 skeletons.
damage dealt by skeletons:
1000*(1-3)-10%=900-2700 damage units or simply put, they kill 22.5-67.5 demons.
As you can see, with these troops, demons kill approximately 36 to 46 percent of skeletons in one hit, while skeletons take the lives of only 8-26 percent of the demon stack.
Even if there are only 200 demons, the damage to skeletons will be 1680-2160 (280-360 pieces), meaning about a third of the skeleton stack will be dead once again :) , which is still more than what 1000 skeletons will kill. Naturally, everything will depend on how you conduct the battle, but imho demons rule, especially in such quantities. So think about it: are demons as harmless as you believe? ;)