OP: post here: https://lists.debian.org/debian-jobs/
I question how one gets here, nay, the 'problem' we're solving.
Presumably there's config management involved already capable of shipping bits. ie: the repo definitions to find these hackjob packages, or deliver them directly.
If this is what you're willing to invest, go Slackware at it - use a tarball. You aren't gaining anything notable by throwing an archive at a packaging format.
The meta that FPM ignores is what provides packages their value! If changelogs were at least a part of it, I'd be a bit more accepting.
Otherwise, I see it mainly as misappropriation. Best case, naive and well served. Worst case, giving the impression of better distribution than actually exists
An organization that does packages, but leaves this as the answer, fails itself and the members. Over 90% of the purpose is dutifully ignored/not standardized
Especially with DEB and RPM, where the packaging format supports arbitrary hooks that run as root, this is a big deal. High quality packages that meet distro standards will inspire confidence in customers' sysadmins. Substandard packaging may do disservice to your core software's brand, if your actual software is more solid and thoughtful than the packaging.
You want people who, when tasked with creating a package for a distribution format that is new to them, look not to tools like this, but to the conventions and standards of successful distros which use that format as examples.
You want 'native' engagement with those packaging formats, not hacks like FPM. The parent commenter's suggestion is an excellent one.
Actual package specs require effort for good reason. It's not just an archive... but interdependence, steps to perform on (un)installation, changelogs, and so on.
There are some helpers already provided, ie: RPM macros.
Sure, they're esoteric, but show me a specialization that isn't. Refer to the Fedora packaging guidelines and enjoy life.
It's for sure a ton harder to use (IMHO) because it mandates the creation of a manifest yaml, versus "fpm -s dir -t deb my-directory && echo tada" but not having to deal with ruby (or docker) can make it a better fit for several circumstances