typelevel/cats-effect

I share that opinion. But, I'm wondering if there are cases where you have a lot of flatmapped actions stacked up on each other in a gigantic IO program, and you know you would like to yield every once in a while, but it's hard to inject that into your code because it's not really clear where those points actually are.

That's the concern exactly. In practice I've found this is rare to the point of non-existent, but I imagine it could happen. The only times I've been bitten by lack of fairness, it was my own fault and relatively easy to fix. If you think about what has to happen for a long series of CPU-hogging code to avoid any async boundaries at all, it usually requires a ton of pure compute code (so, you're doing map with some f which is expensive but pure). In that case, it doesn't matter whether your semantics are auto-yielding or not: it's going to hog the thread.

For there to be code which can be more fairness-optimized by some mechanism in the effect type, but isn't already so optimized, you need a ton of delay actions bound together with flatMaps. A ton of them. Without any async in between and without any shifts to other pools. That… happens… very very very rarely.

Which is to say that auto-yielding isn't as helpful as it sounds in practice. Certainly still meaningful, but more meaningful on paper than in reality.