Here's a filler piece from Slate.
Nina S. Rastogi, Murder, She Wrote: How forensic handwriting identification works, Slate, 26 March 2009.
Here's an interesting standard for expert testimony admissibility:

According to the handful of studies on the subject, a trained examiner will be correct more often than a layman.

One 1997 study
asked both professionals and amateurs to examine 144 pairs of documents
and determine whether the documents in each pair were written by the
same person.

And here's the defense justification for an expert of their own when it's the law enforcement investigator testifying to a handwriting match.  Seemingly this would also affect a common rule that the members are able to make their own judgements about handwriting.  I say this because I've been in courts where the prosecution sought to establish a document as written by the client by offering the document and documents from the service record book "for comparison."

Both groups were as likely to answer correctly when the
documents were, in fact, a match; however, amateurs [oh, let's say CID/NCIS/OSI/CGIS, court members] were six times more
likely to declare a positive match when none existed.