Murder in the first! Crime scene investigations and forensics consider ephemera as sources of evidence. Why would it be any different for genealogists?
Just like crime scene work, genealogists sometimes do a great job. Sometimes we (and they) misunderstand something so that evidence becomes mistakenly applied.
As with any source, ephemera would be categorized to whether it was original or a derivative; the information it contained or represented would be categorized as primary or secondary. Finally as evidence, whether it is direct, indirect or circumstantial evidence would be based on how the evidence was used.1
As a form of artifact (indeed, any source), ephemera faces challenges to its authenticity, condition, identification, provenance, etc. The information contained is subject to interpretation and decipher. These items may be annotated, which begs further questions of identification or interpretation, etc.
In the abstract, there is no reason to consider ephemera somehow superior or inferior to any other source. Among my mothers recipes are those written entirely in Norwegian; some are annotated and signed by aunties.
Genealogy is more than a link to parents, it is about identification and life.
1 This manner of general categorization of sources, information and evidence is modern and widely accepted by genealogists, I believe because it is practical and recognizes how we apply reason and logic to the materials we handle. Some references for comments by others on the topic follow:
(a) Linda Woodward Geiger , CG, CGL, "Skillbuilding: Guidelines for Evaluating Genealogical Resources" , Board for Certification of Genealogists.
(b) Kory L. Meyerink, MLS, AG, FUGA, "Evaluation of Evidence," n.d., ProGenealogists
(c) Elizabeth Shown Mills, Evidence explained: Citing history sources from artifacts to cyberspace, electronic edition (Baltimore, Md: Genealogical Pub. Co., 2007).