Comparing hg branches to git branches in this context isn't super-useful; the right comparison is to hg bookmarks. In this case the semantics seem to be (based on reading the documentation so I might be wrong):

  hg push [dest] -- Push all local bookmarks that match a remote bookmark
  hg push -B [dest] -- Push a specific local bookmark to a remote

To me those semantics don't seem particularly simple or flexible. In fact it seems quite like the semantics git is moving away from but with the restriction that local names and remote names always have to match, which means that you have to be very careful naming your local bookmarks to avoid clashes.