How Multisignature Wallets are affected in case of a hard-fork?
No, multisig addresses are not special in any way that is effected by forks. They are just like any other output on the blockchain.

The reason it is hard for Coinbase is because they need to implement support for signing and creating transactions for those forks which requires significant engineering effort. It's the same reason that they don't support every fork in the normal Coinbase wallet - it is difficult to implement such support so they won't.