Personally, I turn those down. When I'm freelancing, I'm doing it specifically to maintain a high degree of flexibility. Non-scoped jobs take flexibility away.

But I think this is a personal call sort of thing, and may be right for you, depending.

Hourly billing allows that flexibility.

That is illegal and if it ever comes to light you are done

Huh? Staff augmentation contracts are super common. They're definitely not illegal. And the risk in employment classification mistakes is to the employer, not the contractor.

The "you're done" might be an EU thing: I know Europe has all sorts of wild laws around contractor/employee differentiation.

In the US, though, yes, the risk is on whoever the IRS feels is the "employer" in the situation (hint: it's not the contractor).