You are correct to some extent there are some MH and MV ballasts that are the same. In respects to a Mercury vapor in a MH ballast. But a Metal Halide will not work in an old Mercury vapor ballast, they require more voltage to start.
The newer MH lamps do not have a third starting electrode, making them and their ballasts incompatible with mercury vapor.
I have also had problems between brands, same wattage, ansi type, etc. Different lamp manufactuers use different materials for their arc tubes making them incompatible with other manufacturers balasts.
In my experience it is best to stick with kits when retrofitting hid fixtures, preferably prewired ones like Sola, takes out any guess work.