If Ukraine uses ballistic missiles to target Russian infrastructural facilities, then the latter in all probability will retaliate with nuclear strikes. And that will trigger a new world war, where no one is going to win.
While I do agree that Ukraine massively attacking infrastructure on Russian soil wouldn't help ending this war and it would probably make Russia retaliate via tactical nukes if they feel very threatened by it, I am not so sure that it would automatically cause ww3 and nuclear holocaust like everyone is assuming. Its easy to think that US or UK would automatically nuke Russia who would then nuke them, but when you think about it a little bit more, are you sure that they would be willing to sacrifice their almost entire population for the sake of another country? I honestly don't think so.