@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.