I have seen independent projects provide rpms that are supposed to work on both RedHat/Fedora and SLES/openSUSE, with some caveats as to what versions of each are supported/tested for.

Sure. Really depends on, well, the dependencies. The rpm command shipped with all those distributions should be compatible with the format of any recently created package.

But you can run into snags if you have a dependency that doesn't exist on the target system or its repos, or if the packages are named differently, the pre- and post-install commands have assumptions about the system that aren't true on the target distro, etc.

But if all the package exists for is to drop a few binaries on the system, it can be fine. Especially for things like proprietary software that's going to just dump a bunch of stuff into, say, /opt and will only break if the kernel or glibc are spectacularly old or too new, etc.

Sometimes you can even get away with using "alien" or similar to convert Debs to RPMs if it's only packaged as a Debian package. Sometimes.

He he, I tried using alien a looong time ago, without success. In retrospect, I was young and naive, only had a rather vague idea of what I was doing. So I can't say for sure if it was alien's fault or if I was doing something stupid.