I agree with Dan that relative works better. FWIW, I was running into an issue in 10.5.x (wasn't an issue in 10.2.x) where using a network UNC/share-name would stick the local drive letter in front of the path so it wouldn't be found anymore.
if you are talking about an actual addin (i.e. Starting with the wizard to setup the addin) I know once you have the addin installed, you can usually copy the folder with the <ID> to a new machine and it will find everything, if that helps. I was having issues with the .addin file installing in 10.5.1 by just double clicking, but just fully upgrading from 10.3.1 right now, so I just may not have figured out the trick. Addins like these do not work in Pro, but you can usually go to the toolbox and make it work. I also have not included models in my addins, however, in case it helps, I do have a doc that contains some times for Python addins
https://community.esri.com/thread/74093-tip-python-addins-getting-custom-toolstoolbox-to-work-gptool...
It started out as a worksaround and info about that error but I have a few other tips I have discovered. Fwiw.