I personally use Vesktop, which I think is available as a flatpak. My primary reason for finding an alternative discord client was that I couldn't figure out how to get audio working when streaming games in a voice channel; I spent a long time trying to understand the idiosyncrasies of Linux audio before I realised that it was just a Discord problem, and that it was prevalent enough that it had already been fixed by multiple different people.
I believe that overall stability has been better since switching to Vesktop, but it can still be a bit janky. I think that some of this is related to audio problems (possibly a combination of audio and insufficient RAM).