We possibly have different definitions of “showstopper” 
After next month, packages which are updated and don’t require patches can be consumed by having ocaml/opam-repository as an “underlay” (i.e. you have ocaml/opam-repository at a lower priority to fdopen/opam-repository-mingw). Packages which carry patches are going to have figure out upstreaming those patches.
While there have been steps towards it, there has never been an official way to install opam on Windows.