It would be hard to fake unless you had records of all of the mains frequency at the alleged time.
Right, but that isn't exactly secret information, nor is it particularly hard to find after the fact. Just find some other recording that takes place at the time you want to impersonate.
and you will manage to remove original frequencies...
That is a potentially harder problem. Like most forms of digital figerprinting, it is hard to do so without leaving any traces, but it all depends on how sophisticated the analysis is compared to how good the forgery is. For sure a simple automated technique that uses a 2 Hz bandpass filter and a level crossing detector would be easy to fool.
As one tool in a suite of forgery detection tools I imagine it is useful, but it needs to be used carefully.