I'm not sure I understand the question? This is for any package. Of course you will run into trouble if you introduce breakage, like removing files that the build script expects to be there, or causing merge conflicts with patches applied by the build script. But those breakages can happen with any package manifest, including PKGBUILD.
When you say making a debian package is easy I want to know what kind of project it is you have experience packaging, like what type of assets has it got, what technologies does it use. How many languages, how many dependencies, how is it linked? That sort of thing
This is just for arbitrary autotools, cmake and meson projects using any language supported by those build systems. The sizes and dependencies vary. I don't really see it being much harder, the debian developer documentation is really convoluted but that's a different problem.
makepkg -o already applies patches.
And makepkg -e after it is like calling it normally, if you don't touch anything in-between.
The difference there is that debhelper will usually try to keep its own modifications temporary, and otherwise keep the source in the same state as the original upstream source. So that way you know when there is going to be a merge conflict.