We have helped many developers in troubleshooting VSTO add-in load failures. Hamed has written a very nice blog post explaining the basic steps to troubleshoot VSTO add-in load failures. Even if you have not faced this issue yet, I recommend reading the post because it contains basic information about Office add-ins that developers should know.
Although every scenario is unique to itself; there are some common diagnostic steps that are repeated for every investigation. This PowerShell script complements Hamed’s blog and automates the troubleshooting steps; you may find it useful.