It's been 7 months since I didn't write async code so it will be difficult for me to give u a proper answer but I'll try.

the learning curve, the cryptic lifetime compiler errors, the lack of async trait, no async Fn but async block instead, differents libraries using different runtimes, spending hours to compile a trivial code, the box pin Dyn boxfuture thingy, the compile time, issues with analyzer etc.

But most important the fact that you couldn't write code just as you would in sync rust. Most of the features of sync rust don't compose well with async rust. At every step, async rust feels like it's a proof of concept, an half baked feature that should never had reached stable in that state.

Rushing it was a mistake to me. You shouldn't need a PhD in async await to use it properly. It should feel like writing normal rust code. Unfortunately, it's not.