> in what way does this make it "difficult" [...]?
When you get a kernel from Debian, Ubuntu, OpenSuSe, etc. you can see the original source plus a few hundred patches. Each patch addresses a single change, and does so by modifying one or more files. Fictional example: power_management.c, power.h, and core.c.
If you want to see how they changed something in the power_management.c code, you can see which patch modifies this code, and you will also see the modifications to power.h and core.c that were part of this change.
If there were two unrelated changes made in the power_mangement.c code, you'll see two patches and you can choose the one you're interested in.
This makes it easy to make the same change to your distro, or to push the change upstream. Until recently, all distros did this.
Under Red Hat's new system, all changes are lumped together into a single patch. Their single patch will modify 1,000 files. If you see something useful in their power_management.c code, you've no way to see which of the other 1,000 modifications are part of this change (in our example: power.h and core.c), and there's nothing to indicate if two changes to power_management.c are related or not.