We've not had any problems uploading any size of file so far using Shopify's Digital Downloads app. We've been testing files ranging between 5MB and 3GB and no problems uploading or downloading so far. Maybe it has a problem with some browsers, but that would be a question to ask Shopify's dev team. We still need to do remote tests in different browsers and from different parts of the world at once, to see if speeds are fairly consistent for other regions outside the US.
Really, our main concern is whether it can take the load of hundreds of simultaneous orders for 4 or 5 GB files without choking, or without Shopify complaining about us using too much bandwidth. That's definitely something that some web hosts still do even these days. We require terabytes of bandwidth per month and hundreds of GB of storage space on a constant basis, which is why we currently use s3 to host the data and a 3rd party fullfillment service to manage downloads. We're just looking for something more integrated into our shop that users don't need to install an app to use. Free is also a big bonus.
We're likely to use Digital Downloads if further testing proves it's ability to handle the load. I'm just curious as to whether any other shopify stores that move a lot of data have given it a try and what their experiences have been. Not just one-off bug reports and glitches, because any service is going to have them as often as not, even Amazon. We're more interested in the big picture results we might get from heavy use of the service.