clarity of concept

sd-bus supports both the classic dbus1 AF_UNIX transport as well as kdbus as backend. Applications written against the library should in most ways not see any difference. A couple of things won't be available on AF_UNIX though. For example, the credentials concept is much more powerful on kdbus than on AF_UNIX and we cannot emulate that on AF_UNIX.

sd-bus as a library should work fine on non-systemd systems too. A couple of things won't be available then though. For example, it offers APIs to connect to the system bus of local running OS containers, and to system busses of remote systems via SSH.

sd-bus is not portable to other kernels however. We make use of a multitude of Linux APIs, and it's not feasable really to port it, like the rest of systemd. And we will not accept patches for that.

However, this shouldn't really be a problem, as libdbus-1 and gdbus are both powerful libraries that are portable, and can also be used to talk to dbus. If portability to other OSes matters to you you can simply opt for one of those libraries.

Lennart