If you dig into an exacheck output you will find the following check.
Kernel version comparison across cluster
It mentions the following risk.
Potential cluster instability due to kernel version mismatch on cluster nodes.
It is possible that if the kernel versions do not match that some incompatibility
could exist which would make diagnosing problems difficult or bugs fixed in the
later kernel still being present on some nodes but not on others.
It is a best practice to complete the patching in one go.