Multisig makes it so much more difficult. If you can reverse the hash to a pub key and then factor the pub key, that’s 3 difficult problems. Then finding enough of the m of n combos in the entire set of possible keys that creates a single hash output is a whole other problem. When you don’t even know what the m or n values are, you have to brute force every single permutation. We don’t even have a quantum algorithm for that.