Is there a list of add ons that are discouraged from installation, or a list of developers who develop such add ons? If not, is there a list of developers who have been banned from Xenforo? Is there a recognized method of determining which developers Xenforo forum managers should avoid? Should we only purchase add ons that are listed in the Resource section?
Most addons that are legitimately developed for XF do at least list their resources in the resource manager, even if they do not support them here.
If you see an add-on listed elsewhere and not on XF then you should probably check it's from a reputable developer and go through reviews etc as you would any other online purchase/software installation.
I am not aware of developers, where they have their addons not listed in the RM but on external sites.
So just stick with the Resource Manager.
And if you find something you would like to use, just ask on that dedicated thread of that addon or at least read that thread.
If there are problems with an addon, most likely someone else will have mentioned it already.