To be fair, there are some areas where a lefty finish can just about be automatic for many (assuming they have a clean look). Open point blank layups from the left side of the basket would be an example.
That said, there's a big difference between a lefty finish over street folk with a limited contest versus a lefty attempt over Roy Hibbert (or even a DeJuan Blair type) providing a serious amount of pressure at the hoop. I believe the same can be said for lefty jump hooks. Many players have the lefty skill but the majority undoubtedly prefer firing those hooks with their right hand.
In terms of dunking, I would suggest watching any random assortment of highlights and mixtapes and notice how many times a player is able to jam with their off-hand in a game situation. Carlos Boozer is one of the few exceptions I've seen over the years. Elsewise, when players are able to put one down lefty, it's kind of a big deal because it's difficult to pull off (I recall Russell Westbrook being a little giddy about his first in-game lefty flush).
The players with the best chance of dunking with the off-hand are those who elevate off of two feet and even then, again, it's not common that a player can interchange freely. For one footed leapers, they typically have a strong launching leg (for right handers, it'd be the left leg) so if nothing else, a right handed launch can squash their typical exploding ability.