> That can be easily done with comments. If the PR are orthogonal, they could have been split.

Comments are ad-hoc and don't scale, relying on the author to interpret and adhere to the extent of the reviewers approval.

> And if they're not, I would really like to know how the part that I'm reviewing interacts with the rest of the changes.

you are free to look up, down, and around the stack; nobody is hiding the code from you. But in many cases this is just unnecessary.