Strength of warlock lies in a combination of hero power, (arguably best) versatility of cards/archetypes, and unfortunately they have some of the best new cards.
their 1 drops are strong (flame imp OP), they have the best in slot 3 drop in imp gang boss, and given you can orchestrate decks around it, one of the best 4 drops in the game in voidcaller. their hero legendaries are both very good (most classes only have 1 good class legendary) and they have reliable board clears/aoes that not all classes possess
fact that a bunch of different zoos work, from a classic version to a now-defunct undertaker build to facelock zoos to the midrangey version with demons, shows the raw power.
it's insane the best dragon deck is also warlock, given not only does it look nothing like strict handlock, but they dont even use the blackwing tech bc imp gang is flat out better (!)
handlock has been a super strong deck for as long as the game has been around. sometimes you get run off the board by super aggro decks but typically handlocks have good matchups all around.
demonlock, as OP mentioned, is not that popular, but it's mainly because it's basically the 4th or 5th strongest version OF warlock. it is still totally viable and good though.
about the only deck that no longer is good is murlocs. that USED to be a thing.