There are ffbs who are very frum and there are bts who are too. At the same time there are both who are more lax in their yiddishkite. Bts don’t always have this huge high they just live off of. There is a lot of hard work. All else equal of course it is harder to be a bt there is just all more you have to change with a lot less support. If it was eaiser to be frum by being a bit then it would be better to raise your kids not frum to give them a better chance of being frum which doesn’t make any sense. Ffbs could also be Baal teshuvahs in the sense that they could work on themselves and grow.