There’s really no reason to be mad at them in this particular instance. Their client is Chromium-based (Electron) so they will optimize their new features for that engine first. There’s probably less than 5% users who Discord from browser, let alone Firefox, and I think I’m being generous with that number. Additionally, some things are harder to implement (or even impossible) in native web rather than Electron, that has all the NodeJS integrations.
I haven’t used soundboard yet, but I’m pretty sure it isn’t “just” an HTML5 file upload. Perhaps it’s as you said, they run checks on the file being uploaded. Maybe it will work, maybe it will crash in some use cases because they don’t have a polyfill for some specific API they use. So instead of dealing with user complaints about crashes they just disabled the feature.
I’m also not sure why you’re upset with Discord for implementing DRM for uploaded files. If they don’t, they will get sued by the companies enforcing that DRM, so hate on those companies instead.