They're usually just fine. I always try to find a review for a specific AIC, and look at its temps. The most thorough reviewers will run longer tests and see if there is any throttling after a half hour, or so. But typically these run just fine, often with no difference between 2 or 3 fan variants in temps, surprisingly, at the Boost Clock where the board partner chooses to set the card.
It's overclocking, if you ever get into that, where these tend to struggle. But usually anyone seeking a single-fan variant is trying to obtain a card that will fit into a smaller form factor PC, so they're not preoccupied with overclocking, they're trying to keep overall heat output down, anyway.