They're used in the factory.
I am currently running new brake lines on my Jeep from the master cylinder back to the rear axle and they're all bundled together fuel, brake and
vacuum lines using zip ties....from the factory.
Police or inspection agencies are not going to question their use unless they're obviously used in an obviously unsafe manor.
Like fastening your kid's baby seat or the driver's seat to the floorboards with them.
But I would recommend you really fix the problem before losing your Radiator one day....simply due to the fact that strong fasteners are used to
counteract the forces of acceleration and deceleration.
A zip tie isn't made to withstand those types of forces...