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.