My concern is less about it being impossible to legally operate, but that the amount of illegal/spam/malware content that will utilize the service within the free tier will make it impractical to operate.

WeTransfer, Filemail and others seem to be doing fine. But you are right, the overhead of dealing with abuse content might be substantial.

In that case what separates you from them?

Feature-wise?

- Streaming downloads; you don't have to wait for the upload to finish

- Fast uploads; there's a mini benchmark in the background that picks the best upload location

- Verified upload integrity on each file chunk

- Photobooks