Agreed. For media streaming UA sniffing is required because of quirks each browser has in decoding media. Safari has a few quirks about the MP4 packaging it likes via MediaSourceExtensions. Firefox's network stack includes a delay not related to the actual request because of its implementation[0] which makes timing of small downloads very inaccurate. None of these differences are discernible except by UA.

[1] I believe FF includes in the download length the time a request spends sitting in the request queue, but I can't exactly remember.