At least for something like Project Orion, all the individual components are either verified in prototypes or proven technology. We have existing stockpiles of nuclear bombs. Propelling a craft by exploding something behind it is demonstrated in smaller prototypes with TNT. We know how to do radiation shielding, we know how to do shock absorbers.

Or course combining technologies in a new way comes with lots of engineering challenges, but it's a smaller leap in technology than e.g. the first moon landing.