[samus]

For the JVM developers for sure. Implementing Project Loom must have been quite a ride. But even if it is used blindly, there are only three obvious issues I see:

* It's a no-no for computational workloads. As you said, they are concurrent, but not necessarily parallel.

* As you said, care has to be taken to use the right thread when interacting certain low-level APIs.

* It becomes easier to overload upstream systems by sending too many queries concurrently.

[cbsmith]

Oh there's a bunch of other problems as well. Developers will "solve" problems by increasing the number of virtual threads, that actually should be solved in other ways. Tons of code is going to suddenly discover assumptions about its underlying runtime model are no longer true, leading to subtle and potentially complex problems. New software will need to either take on the burden of choosing a runtime model or adopt the complexity from having to consider a mixture of both...