Comment 5 for bug 995113

If any packages fail to download apt should abort without removing any packages, so in this case yes

it should be unlikely to end up in a state were only some packages were downgraded, but if this did happen then ppa-purge is not going to be clean it up later, so in this case (if it can be distinguished from exit codes) could warn the user how to manually find and remove 'foreign' packages and leave the ppa disabled

I suspect ppa-add really meant apt-add-repository, that was in the past buggy when it came to re-enabling ppa's that had been commented out, but that should be fixed these days.