Are launchers as stable and safe to use as stock software? My main concern is that I do not want to modify my Android so that it becomes less stable than it currently is. 

If my understanding is correct launchers like Buzz Launcher or GO Launcher significantly change how Android looks, but the changes perhaps do not stop just there.

Also, once a third party launcher has been installed, is there a way to revert the changes? I mean, sure they can probably be uninstalled, but is it going to be a very clean uninstall, or do they typically leave junk behind around the filesystem?

I'd like to try out Buzz Launcher very much, and GO Launcher probably too, but if they're going to render my Android somehow inferior in terms of causing arbitrary type of damage or just clutter the device as a consequence, I'd rather not.

Update

Incidentally, what about keyboards? Same deal?
In Android, a launcher is no different to any other kind of app. The launcher just has an extra intent filter that launches it when you press the home button. If you have more than one launcher app installed, you get a chooser dialog to select one, just like any other time you perform an action that more than one app can handle (such as sharing a photo).

Android packages aren't like packages in GNU/Linux distributions. They have very limited access to the system, and can't leave files behind them after you uninstall them (except on the SD card, if they request the permission needed to access it). Except when allowed with specific permissions (which you should always check before installing), the only real problem they can cause is using the resources of your phone: taking too much power, network, or storage.

In this respect, you don't need to worry about an unstable launcher any more than any other kind of app you install.


  If my understanding is correct launchers like Buzz Launcher or GO Launcher significantly change how Android looks, but the changes perhaps do not stop just there.


I think you are overstating how much a launcher can do. All it changes is how your home screen looks, including the list of apps and widgets you get to from the home screen. A launcher doesn't affect the appearance of any other app, nor does it change how Android works. It's just an app you launch by pressing the home button.