Is unpublished paper necessarily bad? I am implementing a method from a research paper that was not published in any famous journal and authors didnt state them on their online research profiles. During my assessment, i gained opinion that the method itself is very promising and results section shows good results. I am interested in some opinions on the papers like these. Why is it not published in any famous journals, and was not cited by any other paper?
Unpublished papers are not necessarily bad ones.

How can it happen that good papers remain unpublished? Good papers (or papers with good ideas/methods…) get rejected quite frequently. Reasons may be that the authors targeted the wrong audience or journal, or failed to get their main points across. Sometimes this also happens after a first improvement and second submission and then the "papers-get-old" effect kicks in: Some journals don't want to publish papers for which a two year old preprint exists (have read this from an editor).

Even if the paper is rejected only once, it may happen that the main author has left academia and can't/won't work on a revision and resubmission (and remaining coauthors lack some expertise or time). Even simpler: The authors do not care that much about publication of this specific paper anymore as other research projects got more interesting meanwhile.