Certain clauses will be unenforceable, obviously. But this isn't about enforcing, this is about you granting them permission to do something. After that you'll have really hard time in court trying to argue that they shouldn't have been doing it.
Even in the USA, you can't sell yourself into slavery for the rest of your life, as an example of a contract that might be invalid. Most other countries simply have more consumer rights which disallow even more forms of contract.