When deploying this to live Commons, Roan Kattouw and I noticed that multiple file uploads usually had one mysterious failure.
Roan determined that the records of some uploaded files were disappearing from the $_SESSION record.
Changing UploadWizard's configuration such that it only uploaded one file after another fixed the problem. (maxSimultaneousUploads = 1)
Speculation: there seems to be some race condition with updating, or possibly creating, the $_SESSION data structures that the UploadWizard depends upon. So forcing them to go one-at-a-time fixed the problem. It may be that we should do an initial call to some API to create the data structures for all the uploading files first in one shot, then actually kick off uploads.
Simultaneous uploads are a very useful feature especially for large files, so we really ought to fix this. However it's not a blocker per se.