SimpleMDM support responded to me raising reliability issues with large API uploads with:

The file upload limit using the API is 5GB (10GB using the GUI). Because of how uploads are processed via the API vs. when using the GUI, there are more points of failure that could potentially interfere with the upload process completing successfully, especially for larger files that take longer to upload.

Because of this, it is recommended to upload larger files using the GUI. Larger uploads may not always fail when uploading using the API, so you're not hurting anything by continuing to try but, if larger uploads continue to fail, using the GUI should be successful.

In my experience the API cannot reliably handle uploads larger than 1GB.

Please make the API reliable for any size upload up to the GUI limit of 10GB.