Warlock is an oddity. Easy to pick up and learn, very entertaining, hard to master as a whole.
Obviously Destro is very strong in both areas - and who doesn’t like large explosions and fel fire? Demo while not as strong as Destro, offers a great amount of pressure and hits like a runaway truck.
Affliction. Poor poor affliction. Actually Affliction can preform quite well as a pseudo support peeler for your team. No crazy big numbers though. Dotting the entire field, keeping healers locked in combat with low cost instant cast dots that just keep appearing. SL+dotting healers and mages and MC / Fear melee followed by a barrage of dots
Affliction may not get many KBs, but you’ll provide plenty of annoyance and pressure (long as you’re not being focused). SoC + Taint into groups, often causes them to scatter. Even now, despite the obvious - Affliction can put enough pressure to force certain CDs to be burned early as knee-jerk reaction to allow rest of your team to massacre.
Sticking to just one singular spec does truly limit your potential to truly raise fel. So to speak. But, play what fits you best and what you enjoy most. A be prepared to be focused down… a lot. If the enemy team is smart, they will deny any and all warlocks no matter their ilvl of any spec to just turret. And other days, the other team will wonder why that rando warlock has highest damage and KBs while standing still /lol’ing in mid.