This IPC is a custom built IPC that has no security. This means that any program can do stuff like steal focus or drive other window manager policy if the plugin is there. There is also a plugin that exposes the ability to send key or mouse events.
Applications should just use DBus instead of creating their own custom IPC protocols just because they feel like it.
When I start seeing C++, AUTH, and Kerberos I start getting concerned.
When I don't find a Python-only module for something claimed to be "simple", I start getting very concerned: https://pypi.org/project/dbus-python/
In this specific context it is less about being better and more about being the standard for apps that are part of the Linux desktop except for Wayland which has its own. There are benefits in developers all being familiar with dbus, not having to use different clients for each program you want to talk to, it is easier to secure, etc.
Is it those things?
People don't seem to use DBus at all outside of Linux. That would seem to imply that, by and large, it isn't those things. And the fact that someone on Linux in exactly the situation where it should be used wasn't willing to use it suggests that maybe there are significant issues.
Because it is a part of freedesktop, a project largely about creating software and specifications for the Linux desktop. Windows has COM. Mac / iOS have XPC. Android has Binder.
> And the fact that someone on Linux in exactly the situation where it should be used wasn't willing to use it suggests that maybe there are significant issues.
It is from cultural and educational issues. It isn't just a coincidence that all operating systems I listed above have a standard IPC mechanism that they use for services.
Personally, I prefer IPC because it is simpler to implement, debug and use.