There’s a couple main issues with destro
First, the big issue with demon armor generally revolves with an overall game issue. As we get more gear this xpac our health pools rise, but our damage does not catch up anywhere near as fast as health pools. This has caused every season to be slower than the past. The reason demon armor becomes extra strong here, is the very high health pools in general make it difficult to kill warlocks.
The next issue is a combination of a few other things. Their mastery is quite effective for damage reduction, and the flashpoint trait provided a significant amount of haste. This means that you don’t have to sacrifice anything defensively for high haste levels, which is honestly incredibly useful. Then the damage output combined with tankiness and flashpoint haste makes it relatively easy to get strong benefits from soul leech. Overall you have a spec that can deal a considerable amount of damage, has very high control, and is extremely difficult to kill.
Their powerful self sufficiency also synergizes incredibly well with resto druids or holy paladins, who can chuck out extremely large burst heals from Swiftmend SoTF regrowths or divine favor holy lights. The fact that destro is so tanky means that you can pretty much always survive a CC chain, and your healers can be mana efficient as they can let you drop low knowing they can easily save you - whereas if they had squishier specs on their team they would be pressured vastly more to constantly heal - in which case they can fall behind.
You can hit them in numerous ways. Many suggest flashpoint as a good area to hit. It’s definitely possible, hitting flashpoint prevents those scenarios where you’re just punished so hard for ever coming out in the open to try to win the game. You can chuck out very fast casts on one of the tankiest specs in the game.
I personally am not the biggest fan of hitting flashpoint. I’m actually quite ok with Wizards having very high haste levels and damage/control.
Really my largest issue is the passive tankiness. This kind of forces you to just play to extreme dampening to effectively win against warlock teams no matter who it is and the errors they make. You’re punished for playing offensively ever since they can just counter pressure your go outside of deep dampening in which case they can very easily win the game while you’re stuck in the open because of their fast casts.
The only legitimate weakness to destruction warlock is their lack of mobility, and since mobility is extremely important in high dampening this is what makes it so people can win by pretty much never interacting whatsoever. This makes gameplay pretty unfun on both sides.
Basically, I’m ok with wizards having high damage and control, especially with high haste levels.
However passive tankiness for wizards is always a bad thing, and it lets the advantage of them being ranged shine incredibly hard. You have to sit on them to shut them down, but staying on them also punishes you when they counter pressure offensive goes (which is just going to be reactionary) and it causes - pretty vicious cycle where there’s a person sitting in one spot never moving their character but there’s nothing you can do about it at all EXCEPT wait until 30-40% dampening.
TL;DR hit their tankiness, make other people do more damage to them and they’re fine. Can also just make it so they can keep double coil, crush the passive tankiness and give them mobility with port. Overall that would reward good play much more, prevent the counterplay from being do nothing until 40% damp, and at the same time allow warlocks to still be powerful IF they play well (compared to their being very low personal responsibility as destro on ladder outside of tourney play).
Edit: I’d like to add that multiple high threat spell schools become extremely frustrating to deal with when the caster is tanky. If they die much easier, this is fine because both sides can kind of play around juking/faking and whatnot. Mages also have the same issue when playing fire. But basically flashpoint haste with tankiness leads to two scenarios: warlock spams fear. If you let fears go off your team loses pressure and momentum, and the warlock team can set up bolts on you. If you kick their fear, they also get the opportunity to bolt you for free. Either way you’re in a lose lose. When there is a more consistent threat of death, your DRs are much more valuable and you have to be careful of when you choose to cast vs a scripted rotation where you can sit there as the warlock and do whatever you want and you have an answer no matter what your opponnent does EXCEPT lining until deep damp. WoD mages had this exact issue. BfA fire runs into a similar one (frost doesn’t since they opt out of RoF pmuch, but they still make it so you can never stop them anyways).