Tomohawk has Hidden Power, which has identical BP. No matter what, if it wants that type coverage, it can get it. The fact that Ground is immune to it isn't an issue. If it can learn U-Turn and/or Baton Pass, it already has access to those to moveswitch out of Ground-types, and if not, it can't moveswitch out of Ground-types no matter whether it gets Volt Switch or not. In fact, if it uses Volt Switch frequently for the purpose of moveswitching, that would give Ground-type Pokemon the ability to nullify that effect, and therefore promote them as counters.