The problem, if you want to call it that, is that when you cross two strains (BB and Haze Mist for example) you end up with a mix-matched set of phenotypes within each seed. You may have a handful of seeds, but you wont know if you're getting a sativa or indica dominant plant... Or possibly one seed will grow to be completely packed with crystals and dark green leaves and the one right next to it will be the exact opposite... This is why it takes so many years of back breeding to weed out all of the genes that growers dont want.
Its why it takes so long to create any really solid new strain.
Good luck!!