We have been using sync.com for several years and are quite satisfied. We have not done a lot of sharing from our end but what we have done has worked well. Several bookkeeping clients are using it to share files with us and that sharing is working excellent. What I really like about Sync is that it’s not in your face like OneDrive and Drop Box. You can even set it to not load automatically. Try doing that with OneDrive or Drop Box 
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Harlen