@verita84 Only downsides I can think of is sometimes (not that often) the encode is larger than the original mp4 and some compatibility issues with certain websites (and iPhone users, ppl I know with iPhones can never see webms). With that being said I do prefer webm when uploading them because you generally do get about the same quality at a reduced file size (sometimes significantly reduced) but I do keep the original mp4s for archive purposes.
"While OS 15 has added support for the WebM audio codec in the Safari app, you still can’t play WebM videos on iPhone or iPad’s media player directly. To play WebM videos on iDevices, you need to convert WebM to a format supported by iPhone iPad, such as H.264. Use a third-party converter like Handbrake or WinX Video Converter. They can easily convert WebM in HD, 4K to H.264 or other formats for playback on iPhone, iPad, and other devices."