I've been hesitant to use adds-ons because I don't want to clutter up the software with broken links and unused files or whatever when I uninstall them. Do the add-ons uninstall cleanly or are they messy and slow everything down like windows?
Generally you could install and uninstall as many plugins you want without any real problems. But that is also making the assumption the addon is well written and you do your job right.
Addons have 3 components to clean up. The files have to be deleted the same as they had to be uploaded. Most simple plugins will just have files in the library folder and be named by the company, author or addon. More complex plugins will have more areas to remove files from like js and styles. Just look at the folder from the addon and delete accordingly.
Many addons may not create any tables. But its the one important area that a developer could screw up. An addon that has or modifies table will have install code to create those tables or add columns to existing ones. The addon should have uninstall code which will drop those tables and columns. But not always and part of this can also be because it may not be desirable to lose the data for future use. I am always aware of what tables have been added and since stock tables are probably prefixed xf_ you may see tables prefixed by the developer like merc_ and can drop those if they stayed behind. Extra columns is a bit messy if they did not uninstall properly.
Lastly the xml file you install the addon contains several things. The definitions for the listeners, the templates, phrases, etc. When you uninstall the addon in xenforo all of this is removed automatically. However reality is things can go wrong. They can all be tracked down and deleted in debug mode if something catastrophic happens.
Just run a test site to test addons to see if they are what you want. If they are you shouldn't have any trouble cleaning up behind them later on. As far as the files on the server go you can always upload xenforo and all your current plugins, style files to create a clean file install. Cleaning up the database after years of ignoring it can be achieved using the xf to xf import feature but this won't transfer data from plugins and likely not an ideal option.