Those issues are typically from the decoding libraries you choose. Which could even be ffmpeg if you wish. GStreamer just provides a nice high level API.
It doesn’t mean anything for desktop users, it’s just a new standard that could have wider support than VAAPI since it’s part of Vulkan. Mostly embedded devices lacking VAAPI support today, though nvidia requires a third party implementation so this might improve that situation.
This oversimplifies reality. Fedora has a community and actively makes decisions RHEL has no interest in. But yes they also help with testing many things.
In my personal experience macOS is fine but updates are often buggy or regress performance. Linux has just been the more polished option. Plus it just has a nicer desktop and apps.