apples grafted on saskatoon / service berry last month

apples grafted on saskatoon / service berry last month. September ruby, chestnut crab, Goodland, norkent. So far they’re healthy and growing as fast as anything I grafted on an apple tree this year. There’s at least early indication of good compatibility with these cold hardy cultivars. could this be just an excellent cold hardy dwarfing rootstock? Last winter we had a dozen nights colder than -35C, and the saskatoons are bullet proof to the cold. #applegrafting