Answers

Does Reunion support iCloud? Can I use iCloud to sync/save my family files?

Reunion does not support synchronization or storage using Apple's iCloud.

Reunion uses Dropbox for these purposes. In our experience, Dropbox provides the most reliable, most friendly, and most OS-compatible file-sharing service available.

We are not alone. In January 2016, a panel of Mac experts were questioned about Apple's iCloud (and several other topics). Here is the consensus summary...

The following articles are a little older, but still provide valuable insight...

Article on The Verge

Article from developer Bare Bones Software

Article on iMore.com