We ran everything in one big Kubernetes cluster until a CRD upgrade broke three teams at once. This guide compares one shared cluster, a cluster per environment and a cluster per team on isolation, blast radius, upgrade effort and real cost, with a worked cost model.
A release added a column and our new Pods went Ready and served traffic before the migration Job had even connected to the database. Argo CD applies everything in one wave together. We moved the migration into a PreSync hook, ordered dependencies with sync waves, and show a runnable repro.
Half our Argo CD applications showed OutOfSync seconds after every successful sync, and auto-sync kept fighting the HPA over replica counts. The live objects had fields set by controllers and webhooks that Git never mentioned. Targeted ignoreDifferences and server-side diff fixed it.