In this post, Tomas Wróbel lays out 10 potential drawbacks to the typical PR flows:

1. More long living branches, more merge conflicts
2. The reviewability of a change decreases with size
3. Short feedback loop makes programming fun
4. Reviews tend to be superficial
5. Merging is blocked by remarks that shouldn’t be blocking
6. It’s easier to fix than to explain the fix
7. Developers are slower to adapt the responsibility mindset
8. PRs discourage continuous refactoring
9. Negative emotions and outright pathology
10. How do you switch to branches with migrations