What's the point of doujinshi? Going through the pages, its mostly scans, some comics, some magazine scans/artbooks, and a couple of series pools. There doesn't really seem to be unifying theme to all of them. The wiki page mentions webcomics, flash games, and animations but almost none of those actually have the tag. I've uploaded several comics now without the tag because I've never been really sure what it meant.
If you are looking for comics, then you search comic. I know a lot of people already blacklist comic because of random dumps.
If you are looking for magazine scans, then you search magazine scan.
If you are looking for any fan made work, then you search -official_art.
If you want anything that isn't covered by comic, you search the pools.
Doing -doujin_cover -pool:series -monochrome -greyscale -artbook -comic doujinshi shows pretty much no correlation between the images, just that most of them have exhentai or e-hentai as the source. I would add -source:*e-hentai* and -source:*exhentai* but it times out after that.
If anyone can offer some explanation (and maybe a wiki rewrite) of the tag, it would be appreciated. Do all comics count as doujinshi? Does the comic have to be physically scanned? Plenty of artists post old doujins to pixiv. Do the cover pages all count as doujinshi? I was thinking about a BUR, but there's too much under the tag to be changed.