I found Discipline to be somewhat usable for leveling and soloing in Legion and in BfA for Priests since it's now supposed to be healer/dps hybrid so it does have some damage and if you pick all the talents that give you maxium amount of damage and always pick the gear that gives you bigger damage etc. and then abuse the survivability for the game content. It also uses both shadow and holy spells so getting locked out of schools is not big issue.
For warlocks destruction and demonology both have nice amount of spells for both fire and shadow schools so lockdowns are not much of an issue but Affiction is bit vulnerable. It's best just try to time your hard casting when there no mobs in melee range and then use only instants if you have mobs on you.