Any reason for the grouping? Sometimes deps are related and must be updated together, but other times one update is problematic and it will block the others.

Mainly to reduce the total number of PRs to reduce "alert fatigue". I and many others get lots of PR/issue notifications, and these multiply on Dependabot days.

Other projects I maintain use grouped updates, and very occasionally one update may be problematic, but like you say, it goes both ways and sometimes they need grouping.

The vast majority of the time, grouped updates are fine.

Multiple PRs use up more CI time, and are also more cumbersome: after merging one, often it needs to rebase all the others, using more CI time, and needing to wait before merge.