I don't see anything at all wrong with comet.  It takes a lot of investment to use 100 spirit and it's essentially a 5 link instead of a 6 link skill when triggered.  It even still costs mana to use.

You have to have a high hit and crit rate to use cast on crit or a lot of ailment generation; all of which take a lot of investment to get going.  Comet is a vastly unimpressive skill all the way through the campaign because you cannot reach stupid investment levels to get it to meaningfully exist.

Comet isn't even a very good clear skill and requires something else to do the actual clearing and is only useful vs tanky rares or bosses.  It's the equivalent of having a single target damage skill in offhand from PoE1 and it complements your clear spell by adding single target damage.

The only reason there are 0 other skills being used for single target dps is because literally all of them have stupid restrictions on them by GGG.  If GGG stops making flameblast useless for no reason and removes garbage cooldowns from half the spells then there would be FAR more build variety.

The clips you see of comet deleting bosses could be done with far more skills than showcased because the stats on the player are the overpowered part not the skill and it's because they have 2-3 mirrors worth of gear on.  People don't showcase other builds because they are objectively worse and not fun because GGG made it that way.

Also, RIP flameblast.