My experience with asynchronous programming is brief, but it taught me that you will never have elegant parallelism. It's a completely different problemset, you need a completely different solution to handle it. But the ideal API wants to treat the problems as one in the same.
So the two camps I've seen on solving it is either
a) blackbox it and set some rules for implementers, inevitably leaving to some hidden and very nasty bugs on the user end or bottlnecks for advanced users, or
b) give implementers the full tools to setup, launch, and synchronize themselves, which may end up being slower than a single threaded solution if the user isn't adept already with parallel programming.
You inevitably always have bits of a) creep in, even for the most explicit solutions
So the two camps I've seen on solving it is either
a) blackbox it and set some rules for implementers, inevitably leaving to some hidden and very nasty bugs on the user end or bottlnecks for advanced users, or
b) give implementers the full tools to setup, launch, and synchronize themselves, which may end up being slower than a single threaded solution if the user isn't adept already with parallel programming.
You inevitably always have bits of a) creep in, even for the most explicit solutions