Donk bets exist in the meta, ie when the turn is extremely good for your range but is horrible for your opponent. ( if you have a fd on the flop and it hits on the turn you can overbet the pot on the turn with your bluffs and foushes then just go all in on the river) if they have top pair its pretty hard to play against that
Yea I'm not saying it's impossible to devise an unexploitable flop donking strategy. I think the reason thinking players generally don't is because of the complexity of adding significantly more branches early in the game tree - basically going from 3 (check-{fold,call,raise}) to 6 (those 3 plus donk-{fold,call,raise}).
The other issue is that increasing the number of branches also decreases the number of hands that go into each bucket, to the point where it might not be effective any more without being able to randomize the branch choice for specific threshold hands. Most pros I know just have a hard cutoff for each branch and don't worry too much if they're slightly out of balance, but smaller bucket sizes could magnify errors. If you have 31 combos for one action when you're supposed to have 30.5, then whatever, but if you have 6 when it should be 5.5, that could become a problem faster.
