I’ve been using resources quite a lot since Rapid Weaver 8 was released and have never had a problem.
As Don @DLH said, there have been lots of people who are or have been using Dropbox that have been reporting corrupted project files.
That’s essentially what the missing resources message is, a type of corrupt project file. Your RW project file is actually a special kind of folder called a package that consists of a bunch of other files that make up the “project”. Any Irregularities in this folder can make the project unusable.
This Dropbox corruption has happened to people weather they use resources or not.
More can be found on the forum, most people are switching away from Dropbox to iCloud or other services.