There a few minor UX flaws that make it frustrating to use, e.g. having to set Docker host, shared filesystem performance is poor, networking in enterprise desktop environment is broken (just to name a few top most issues).
Also, a lot of folks end up running Docker for Mac and minikube VMs, why should they have to run two VMs?
Additionally, minikube is completely different from production-grade deployments (single binary, which means a rewrite of main function for etcd and all control plane components, as well as hard to debug basic performance issues in control plane, there is one large process and you don't know what is wrong, also there is no way to use your favourite network add-on).
Additionally, minikube is based on legacy Docker libmachine, it is not really maintained anymore.