Honestly I'd only think of running Mongo on Kubernetes if my organization is thoroughly bored with how well they've managed Mongo upgrades, scaling, outages, migrations, and tuning over the last year on a traditional VM setup.
Even so, if everyone's bored with the stability, why change it?
I'm an advocate for databases in dev/test environments on K8s due to ease of deployment, but there's too many moving pieces around storage for it to be a great idea for production out the gate.
Well. The benefits (monitoring, telemetry/observability) are nice enough that I might think about running it on k8s in prod. But only after sufficient number of dev/staging clusters have been sacrificed.
The API is so slick that it makes you feel you can do anything. But there's can, and there's should.