I've had an issue with an LG smart TV (not really an LG, its a re-brand). The web browser was horrifically slow and it flat out refused to play anything. It has very limited processing resources.
I was planning on side loading the jellyfin client for LG TV's but that was all dropped in favour of a chromecast. Instantly solved the problem (and they're quite cheap on social media marketplaces)
Other than that, I've never had an issue with any client.
My media is a mix of pretty much everything available. 264, 265, avi etc etc.
I occasionally have an issue with 265 media, stuttering. Easily solved by deleting and redownloading a 264 version.
Transcoding (or not, dependant on client) is all handled automatically by Jellyfin.
Yes. Bedfordshire.