I think the comparisons fly around a bit everywhere.
Hypothetically and arbitrarily speaking, if a Demon Hunter and Warlock were equally good, and the same average-skilled player had to play both, then that player would probably achieve slightly more success on the Demon Hunter.
Because the skill-floor is lower on the Demon Hunter.
It’s generally perceived that melee dps specs are easier to play adequately than caster dps specs.
The mechanics are more straightforward, there’s less finesse to the gameplay, and the playstyle fits the strategies.
There’s no cast bar to account for, the rotation doesn’t deviate much from the design intent, and the Arena style of focusing on a single target fits a class design where the strength lies in a single-target focus.
That’s in contrast to the caster dps where the mechanics are a bit more complicated, it requires a little more finesse to play well, and the playstyle is often shoehorned to fit the strategy.
Prosaically speaking, then it’s easier to press Chaos Strike than it is to press Shadow Bolt.
It’s easier to dps a single target than to multi-dot several targets.
It’s easier to try and stick to melee distance than to try and stick to range distance.
And so forth.
Skill-floor inadequacies don’t mean much to the top players and their performance, because they can play everything to its skill-ceiling anyway. So long as the skill-ceiling is balanced across all the specs, then the skill-floor doesn’t matter to them.
But for more average-skilled players, for whom reaching the skill-ceiling on a spec is not realistic, what determines their performance is more the skill-floor balance.
For those players the spec that is really powerful but really hard to play is less likely to give them good results versus the decently-powerful spec that is super easy to play.
Because the skill-floor matters more than the skill-ceiling for the vast majority of players.
And that’s why there’s a prominence of specs with a low skill-floor. They’re just easier to perform adequately with. That’s way more appealing to people than a spec that is super powerful but also so difficult to play that they can never pull it off.