You can see a full run down of X7 on Sierra on my post here: http://community.thomsonreuters.com/t5/EndNote-General/Attention-Mac-users-X7-PDF-viewer-and-Sierra/td-p/131351
If you are a Mac user that relies heavily on PDF annotation workflows in X7, we recommend that you do not upgrade to Sierra. And, if you do, and encounter an issue outside of PDF thumbnails missing, and misaligned annotations, please get in touch with our Support team.
Thank you,
Meredith M.