We're currently dedicating 4 nodes (ganeti VMs with 15 vCPUs and 5GB memory each) per DC to run only sessionstore/kask.
AIUI the initial reasoning was that the services does process PII and, during the early days of kubernetes adoption, we felt the need for increased isolation by not running it side-by-side with other containers.
With most of our services now running on kubernetes, the higher level of confidence and the improved tooling around it it might be time to revisit this decision. Removing this snowflake would reduce complexity and a point of failure (which already caused an outage at least once).
I could not find documentaion about the decision process to run on dedicated nodes. If somebody happens to recall details, please share.