k8s is just a bunch of processes working together. The downside is that if something goes wrong, it is unclear which process is faulty and how to fix it. A misconfigured DNS may take down the whole cluster, and the symptoms are that every process fails because the network is out. Difficult to backtrack to the fault source, there are a lot of configs pertaining to the network that could be faulty.

On the plus side, if someone else configured the cluster correctly for you (e.g. a Cloud service like GKE), then it's a breeze to use.