Checkmk does not offer HA at all, the Appliance only have rudimentary support for some kind HA, but not really a cluster.
We have been running Checkmk in Active/Passive with just keepalived and rsync (more or less) for years and I find it working ok.
This works especially well during upgrades or either the OS or Checkmk where we can just failover from one node to the other, run all upgrades and fail back.