As far as I know, io_uring is quite secure: a user cannot perform a syscall through it unless it has the privileges required to perform this syscall directly

I would gladly get more details about the exact purpose of seccomp in a container environment. Reading a bit of internet, I find that docker "uses seccomp to block mount(2), which could be used to escape the container", which makes no sense to me because mount(2) requires CAP_SYS_ADMIN

io_uring cve's: https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=io_uring

seccomp is used for defense in depth. If someone managed to escalate privileges through some means the seccomp policy will still prevent them from doing nasty things or escalating further.

That's not contradictory. Capabilities in docker are also limited, but both are used as a part of defense in depth.

That would be impossible to know. The main thing with io_uring is it makes it so you don't need to context switch (ie make system calls) to perform a number of operations.