Years ago, yeah. It took some time to figure out the new .kfx format. Now? If you're on a macOS/Windows no setup is necessary, provided you have the Kindle app installed. If you have a physical Kindle and you use Linux, the setup is as easy as looking up a serial number in settings and entering it into the plugin's settings. If you don't have a physical one and no access to a Windows machine, then it is a bit tricky, yeah. That said, I assume this doesn't apply to at least 70% of its users.

As for the plugin's stability, I have yet to experience a problem after like 7-8 years of usage. I've upgraded from Calibre v4 to v5 before the plugin supported v5, but that's about it.