I did some experimenting and determined the cause of the upload stalling, at least for me, was multi-page receipts. Any time I uploaded a single page, it immediately asked to confirm the amount, and then went into uploading as usual (less than a minute to process). When I attempted to upload multiple pages, the amount confirmation never popped up, and it started the endless upload cycle instead, taking over a few hours. I tried this on the same receipt several times and repeated the issue.
This is a real pain especially if you have a second page clarifying the difference paid in different currencies, or want to attach a credit card confirmation! For now, I'll only use ExpenseIt on Concur for one page receipts, and will hold on to the additional pages for auditing purposes. If totally needed, one can always do it the old fashioned way.
I'll also note that I found it makes a world of difference to use a darker background, and to keep the little rectangular auto-select option on.
