Do Rancher side products generally make it into a stable state such that you would want to run mission-critical systems on?

(Former employee) They tend to either get enough traction very quickly and be supported for years, or not and be abandoned in weeks/months.

RKE (their Kubernetes deployment and management platform, mostly for various flavours of self managed environments) is pretty popular with the self-managed crowd that needs something to manage their on Orem Kubernetes clusters.