Tradeoff is inability to run on anything besides recent Linux kernels
I'm not hip to how much new stuff is backport-able, so this may preclude Ubuntu 20.04, for instance. You lose the "LTS" part if you compile your own kernel, if you manage to make it functional at all.
Note: I never use kernel modules due to the issues rhel/debian and I have had with such things in the distant past.
I'm not sure that concern is justified. It seems io_uring was pushed as part of the 5.1 linux kernel release, and Ubuntu 20.04 LTS seems to have been shipped with 5.4.
https://packages.ubuntu.com/search?keywords=linux-image-gene...
Also, a quick Google search pointed to io_uring patches for Ubuntu 18.04 LTS.
It does sound like extendible hashing might have downsides in some scenarios also.