Are the upgrades seamless?

I'm new to openshift and I heard some stories that for instance, when you upgrade Openshift Container Platform, there are usually issues with the EFK stack that is deployed on top.

This comes from the fact that the OCP upgrade procedure is not aware of the running application on top.

(I didn't verify it myself, but this is just to give a context to my question, and if it is not the case, then the question is still relevant)

My question is then, what about CNS? Do we have successful stories of customers upgrading from say OCP 3.7 to 3.9 with more than 5 glusterfs nodes and with many pods using CNS without any downtime? And if yes, what are the recommendations in the planning to avoid downtime during the upgrade.