Sap drops off hard when enemies have multiple attacks, since it only ever affects one attack.
I have used it pretty effectively until level 20, especially with a Fighter and ranged weapons. Also this presumes you attack the same enemy twice, which you do a lot, but not always and it offers flexibility in application.
It's also completely worthless if an enemy doesn't need to rely on attack rolls.
Sure, but they are all situational. Graze is rarely useful on a very high level Fighter, or at any level against an enemy with very low AC. Vex is worthless on enemies you kill with 1 hit (most enemies at 1st and 2nd level) or where you won't be attacking the same one on multiple turns. Cleave is worthless if there is not another enemy within 5 feet. Topple is actually counterproductive if your allies are using reach or ranged weapons. Slow is worthless if you are already in melee with an enemy and neither of you are leaving. Push is worthless if you don't want to move the enemy. Nick is not useful unless your bonus action is being used for something else.
Did I miss one?
Compared to the situational nature of all the others I think sap is pretty effective and I think is useful in more situations than just about any of the others, except maybe nick. There are times causing disadvantage on one attack won't be useful at all, but that is rare. I think the others, except nick, will come into play less. Some of them will generally be more effective when they do come into play though.
On top of that, the only build that would ever actually use Sap—the aforementioned sword-and-board-type character—benefits much less from Sap anyway because they're already hard to hit.
This is not true. Disadvantage is more debilitating with a high AC than with a low AC, moreover the disadvantage applies regardless of who they attack.
It's not even good for a tanking build because getting into melee and imposing Sap means the target has no incentive to attack the character in melee over someone more vulnerable at range, since their attack is at disadvantage anyway.
I would disagree with this too, for several reasons. To start with ranged damage, when enemies have that option, is generally lower than melee damage, generally imposes fewer bad conditions and often affords fewer attacks. Second your allies often have cover if you are in melee with an enemy.