This — plus, it's often kicking the can down the road: All the different factions in Congress might agree that something needs to be done, but they can't come up with agreement on the details (often because of special-interest lobbying), so they say, "let's get a bill passed and let the agencies deal with it, and if a major problem comes up with a particular agency ruling, we can revisit the issue then."
My understanding is that the big problem is that this expectation is implicit.
Congress isn't granting agencies the authority to make determinations as to what falls into a category, for instance. Instead, they're creating a category without further elaboration.
Compare that to 18 USC 921(a)(4)(C), which says in part:
The term “destructive device” shall not include any device which is neither designed nor redesigned for use as a weapon; any device, although originally designed for use as a weapon, which is redesigned for use as a signaling, pyrotechnic, line throwing, safety, or similar device; surplus ordnance sold, loaned, or given by the Secretary of the Army pursuant to the provisions of section 7684(2), 7685, or 7686 of title 10; or any other device which the Attorney General finds is not likely to be used as a weapon, is an antique, or is a rifle which the owner intends to use solely for sporting, recreational or cultural purposes.
My understanding is that this would not be impacted by overturning Chevron, as the process was established by Congress explicitly. Hypothetically, if that provision did not exist and the AG unilaterally decided that a weapon that was otherwise included in the category should not be, then that would be an example of executive rulemaking within the bounds of Chevron.
I wonder if going forward congress can just try to have those agencies iron out the ambiguities before passing the law? Or is the idea that its impossible to anticipate all possible edge cases and congress wants to let the agency iron out future issue?