The last time I asked the same question here, user dwattttt finally pointed out[1][2] to me that there is a significant difference: wfmo can actually acquire semaphores in addition to waiting for them, which poll can't do in a non-racy way and efficient way. It can also do rendezvous synchronization (i.e. signal-and-wait).

[1] https://news.ycombinator.com/item?id=47513667 [2] https://lore.kernel.org/lkml/f4cc1a38-1441-62f8-47e4-0c67f5a...