The two CR7 giants both have a +9, so to hit on a 5 your melee combatants need to only have 14 AC in Tier two. Heavy armor types like STR fighters, Paladins, and melee clerics should have at least splint by then, giving them 17/19. Dex types should be around 15-16 (Studded leather plus 3-4 from Dex). Medium armor types should be 16-17 if they do a +2 dex and shield. Moon druids and barbarians will probably be in that 14-15 range since they rely on resistance or form HP, while Ranged attackers and casters should endeavor to stay out of melee. General buffs and reaction spells like shield can mess with this too, but I think it's fair not to worry about an EK rocking 24 AC for this comparison. When you take the +9 up against AC17, it's still quite effective (hitting on an 8), but that's a 65% chance of a hit normally vs a 42% with disad - it's not the drop from 50 to 25 that you get at 11, but I'd say it's still a pretty effective reduction. Down at 5 it's 80% normal 64% with disad, where I'd agree that it's not overly effective. As the long-running 11 vs 20 argument shows there's a lot of variation in opinions of how good disadvantage is, but I think that it's not unreasonable for a party to have good enough ACs in melee that disadvantage is still effective even with +9-10 attack bonus.The problem that I've found in practice when using Vicious Mockery against big, dumb heavy hitters - which is where you'd think it would be the most effective - (such as the zombie t-rex in ToA) is that while they're likely to fail the Wis save, they're also likely to land attacks even with disadvantage due to really heavy melee attack bonuses (like +10). Disadvantage isn't that great against something that is going to connect with an attack roll of like 5 or higher. Basically, I do think it's a pretty great cantrip in tier one, but it hits the wall HARD at tier two.