BackBlaze was not alone (not sure who was first) to mount drives vertically. Xyratex did that in the near-line storage units (e.g. RS-4835), as did Hitachi and others. Once you've seen them, it's the obvious choice to maximize the number of 3.5" drives in a rack (~48drives in a 4U chassis). Heavy beasts though -- you want a lift to mount them.

Don't know about relative reliability to horizontal mounts, but afair the drive manufacturer recommends mounting either vertical or horizontal and we didn't observe any obvious issues.

BackBlaze publishes good data, although I suppose that other manufacturers (such as Dell) make storage appliances with vertically mounted drives thats a data point that they at least think its okay-ish.

If Andy is watching this thread, I'd love to hear if he explored this topic previously.

For what it's worth, a bunch of server OEMs sell top-loading storage servers, eg. https://www.servethehome.com/dell-emc-poweredge-xe7100-100-d...

Please, Backblaze, for science, install some racks lying on their side. :)