The reasoning is that drives are produced and shipped in batches and if you order multiple at onces there is a higher chance you’ll get drives from the same batch. If that batch had some fault during production or it was damaged during shipping, all your drives might be affected.
I don’t have a source, but it’s something multiple expirenced people have mentioned to me.
After I had two WD drives fail in my old NAS so I switched to all Seagate on my next build. Currently running 9x 20TB Exos X20, though for only about a year now, so no issues should be expected, yet.
I think the most important thing is that you pick a drive that is meant for NAS/server use (so rated for running 24/7). And having manufacturere warrenty is also nice. My Seagate drives have 60 months (which is considerably more then the 36 months that my WD drives had).