A limitation I ran across when using B2 was that their pre-defined url generation doesn't allow you to set file-size limits nor does it allow you to set the file name in the pre-defined url. It simply gives you a url to upload it to. So if you are using b2 for storage for lets say image uploads from browser, some malicious user has the ability to modify the network request with whatever file name or file size they want. Next thing you know, you have a 5gb sized image uploads happening....

This pretty much prevents me from using B2 for now.