2/ Ambiguous build failure error message or segfault. 

Here I use the shell option to go into to chroot. It has some quirks as well. It drops you into the root so you have to do the whole cd builddir/build/BUILD/... or something like that (I'm at $DAYJOB right now). Could it not take you directly to the build dir?

This is good point actually. I do this all the time. I have opened this upstream ticket:

https://github.com/rpm-software-management/mock/issues/693

Also useful tools like vim or less need to be explicitly installed and often don't work exactly the same inside the chroot as they do outside. BUT it does allow you to interrogate/troubleshoot binaries and test, etc.

While I typically tend to use editor from my host (I quite often use GVim or GEdit, which are both GUI editors), I stumble upon the missing `less` quite often. If there was way to somehow `mount` the editor from host into the buildroot, but I can't think of any feasible way :(

I have already withdrew the original proposal, but that does not mean I
am less concerned.

I don't think it's a waste. I agree that we should encourage use of mock/fedpkg mockbuild in the documentation but we could also try to remove/reduce the reasons people use fedpkg local.

Thank you

Vít