are birth control pills prescribed for acne as effective as the others at preventing pregnancy ? or is the dosage of the hormones too low?

Yes they are just as effective. Birth control either works or it doesn’t, there is no “percentage”, the only reason why it states as a percentage is because of a genetic mutation which makes birth control not work in the first place. This isn’t something that develops, but something you’re born with and it’s incredibly rare.

Any birthcontrol prescribed for acne is no different than birthcontrol prescribed for contraceptives. There is just certain ones that may work a little better at reducing acne than others but there’s no difference contraceptive wise