The intent of the acceptance sign-off is that the person doing it can be readily identified. The standard talks about signatures, but what is meant by this is that a unique and clear identification method is required; yes, signatures are unique and have been used for years as a means of proving that the signer is the owner of a signature, but they often cannot be read - at best, they're used by comparison to a known sample that are alongside a printed name or photo. Banks still do this. Look at the back of your credit cards - we've been taught that the more cryptic the signature is, the better. Not so for the records in question at all! For records, we're not worried about forgery, we're worried about lack of legibility.
Well, legally, yes and no.
Sometimes when FDA uses the word "signature", they are using it in its legal sense. A "signature" is a legal commitment by the signer that they read, or did, or agreed to whatever's contained in the document they have signed.
That's why a bank check requires a signature, and the bank won't be happy if you use a rubber stamp unless you've arranged with them in advance to their satisfaction that the particular stamp you want to use legally comprises a signature. Ditto with a prescription form and a rubber stamped signature.
And note that in regard to the legal significance of a signature, readability isn't relevant. Many doctors' signatures are completely illegible. They must teach that in medical school.
This metaphysical-legal significance is why 21CFR 11, and FDA in general when they're focused on responsible approvals, makes such a big deal about signatures.
Initials--actually, a recognizably personal initial-object, not just the right set of two or three letters affixed in some manner other than handwriting--sometimes can be a legal signature, if they're so intended and that's understood to all concerned parties.
The second best approach to the use of initials in an organization that has FDA obligations for record signature is to have a signature card or log for each individual that may need to sign an FDA required record, and have each such individual's script signature and recognizably personal initial-object on the card or log. That's the approach that is used where I work. The first best way is to have such a signature card or log, and limit everyone to the same full signature that they would use to sign a check.
Other times, the above quoted analysis is correct, and the point of a signoff is just to identify who did the work. In those cases, a rubber stamp is fine.