I don't even think that they would always be equally as difficult, it depends on whether the person in the SAC wants a wide spread of scores or a narrow spread. Unless the teacher writing the SAC was the same in both subjects, it's likely they would vary in difficulty. But that also depends on the school...
Even for subjects like chem, I've heard my friends at uni talk about how they always wrote up reports for prac SACs whereas at my school, we had short tests after a prac and didn't write a lab report. There is certain stuff a teacher has to cover in a SAC but the way they choose to do it varies between schools.
So your sacs might be easy but mine might be hard. Who is to say? We have not done each others SACs.