>I'm pretty certain that national law takes precedence over what someone put in a contract.

Yes but only if the law says that you can't create a contact that signs away that right.

For example, in the USA you can reverse engineer. Totally legal. But you can also sign away your right to reverse engineer. That is what a contract is, signing away your rights.

But the US could also pass a law saying it's illegal for a EULA to prevent reverse engineering.

So just finding a law that says reverse engineering is legal, doesn't mean a court won't hold you to a contract that prevents reverse engineering.

That said, it's probable that some countries have banned contracts that prevent reverse engineering.

Another example for tenancy is that your landlord can not prohibit you from keeping small pets[0] (including cats) even if you sign it.

Another example is that EULAs are prohibited by law from containing any clauses that a customer could not reasonable be expected to assume to be in them. This hit WhatsApp when it tried to ban users who used unofficial clients (which would not violate the EULA of most other messaging services and can therefore not be prohibited simply by adding a clause to the EULA).

[0]: within reason, obviously. The landlord would have to prove that the pets present an unreasonable nuisance to your neighbours or cause unreasonable amounts of damage to the landlord's property.