What are the downsides of using Postgres or MySQL for K8s control plane?

I've setup several k8s clusters on bare metal in the last few years, and by far the biggest challenges are storage and control plane corruption.

The last one is usually due to multiple etcd failure modes (corruption, daemon/pod not starting, network/port issues, ...) to the point where even with backups (restic, velero, etc) the cluster needs to be rebuilt.

So I'm considering using an RDBMS outside the cluster. Assuming I can guarantee availability and performance, what are the downsides of swapping etcd with a relational DB?

Thanks in advance.