Arbitrary rule changes. Definition changes. Things that they have held as legal for years can become illegal with a change of interpretation of existing legislation.

With how political the regulation of guns is, the broad authority to easily change the rules without underlying change in the legislation results in a highly politicized agency.

One blatant example would be treating ar15 lower receiver as firearm (ATF rules).
According to the US Code of Federal Regulations (aka the law), a firearm frame or receiver is defined as: “That part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel.” which lower receiver ain't. Together with upper receiver (where the barrel goes) it is a firearm, but lower by itself it's not. A lot of people went to jail for selling a part of the gun which ATF capriciously (not legally) classified as a firearm. The list is long...

You must hate the second amendment