This is specifically not the case - loan sharks are frequently punished for creating contracts that are not reasonable, and cannot be upheld by law as it violates consumer protections. One of which is that contracts are required to be reasonably understood by all participating parties.
BNPL providers are held to far higher standards than Credit Card, Personal Loan, or Payday Loan companies. I'm not sure I understand the HN addiction to attacking them.
Unlike all the examples above, Afterpay doesn't charge interest, has capped late payment fees, and has a very low debt ceiling. It's the perfect way for younger people to learn how to manage debt.
If a young person is learning to manage debt with them, that necessarily implies that they don't yet comprehend what taking debt means and cannot reasonably be expected to fully understand the consequences - which may make the contract invalid.
This definitely isn't enough to make the contract invalid, however you could prove every BNBL business wrong and put them out of business if you find a court that agrees with you.