In sqlserver (and others?) you can have one clustered index. Typically this is the primary key (doesn’t have to be though).

The clustered index is actually the physical order of the data stored on disk, so any new guid causes tremendous amounts of churn.

Not sure how modern this concern is, but I would guess a LOT of SASS companies have to consider this.

Quick edit: sequential GUIDs are obviously a thing and I believe alleviate a lot of these concerns. I do not believe that was an option in sql server 2008 r2 (a version that lived a LONG time).