Of the 10 rainbows I have (not counting Sakura dupes), only two of them are from rolling FGs. And I've rolled a lot (though not several hundred dollars' worth in a week like some people).

You really do need some crazy luck if you don't invest in the guaranteed deals.

You don't need 6stars to do any of the content in the game right now, they're mostly just for looks. 5star teams can complete everything and after playing for a few months I have enough fivestars to fill every team (but still need to level half of them, rip).

I got my only 6star by cashing on the black friday deal. If you want 6-stars any other way you'll have to, on average, drop 900 flower gems per 6star on 10+1 pulls. While I'm certain people have done that, I wouldn't hold my breath waiting for it to happen.