What a great video! The look of accomplishment on her face is just priceless...
My daughter hated tummy-time so we never did it (much to the dismay of her doctor and many other well meaning relatives). It didn't delay her development in any way... she learned to roll back and forth just fine and was power-crawling everywhere way before she was expected to.
Each baby is different. Whether they start out on their tummies or their backs, learning to roll over will be a "struggle" as will so many other things that they learn. I think of it's a very important part of their development.