ReiserFS does have its benefits in highly specific use-cases.

I have a filesystem with an absurd number of tiny files in it. I host a statically rendered wikipedia mirror. Tens of millions of gzipped html-files with a filesize in the range 1-5 Kb.

ReiserFS is the only filesystem I know that deals even the slightest bit gracefully with this thanks to tail-packing.

Wouldn’t it be better to keep such files inside a database?

https://www.sqlite.org/fasterthanfs.html They're selling it quite well here, though integration into other products might be the hard thing.