DroboShare shares fine, but your intended use - video editing, is not a good match for DroboShare.
DroboShare (according to DRI) is designed for “office use” meaning shared documents (documents, spreadsheets, etc).
Video editing requires high-throughput, direct-attached storage, unless you spend LOTS more money on a SAN solution.
Directly-attached Drobo should work OK for your use unless you’re editing high-bandwidth files or more than two streams of video, which I doubt is the case with iMovie.
You’d have the same problem with iMovie and any non-direct-attached drive, as it seems to not allow for network shares as video/project storage.