Amazing how many people think that this would be a good thing.
If the bill forces Apple to give 3rd party service depots the same tools that Apple has for servicing / diagnostics, then, yes, there's certainly security concerns here - you'll see more IMEI cloning, more ways for the authorities to get around your secure enclave. They'll make iPhones much less secure.
You also really can't expect Apple to allow this to happen for in-warranty repair either. Some fat-thumbed idiot leaves a loose screw inside after replacing a component and it shorts out the system board, but now Apple's on the hook for a warranty replacement?
People also equating this to other things have obviously never repaired any phones. Many, many, very delicate, very tightly packed, miniaturized components. There really isn't anything comparable in terms of how tight the confines are.
Maybe a good analogy would be, would you like your GP to also do brain surgery on you?
This will result in two things if it becomes commonplace throughout the US and the world:
1) Apple's warranty costs will go up, maintenance revenue will go down, so the cost of iPhones will go up.
2) Longer term, the phones will become fatter and heavier, as the manufacturer's will need to change the design so that almost anyone can service the phones with a minimal amount of skill. Expect everything to be compartmentalized.
eg:
https://phonebloks.com