Yeah, seeking is still a bit flaky. But subtitles work great for me, at least when using the Web and Roku clients. I got a better experience in some corner cases on the Roku client by making sure that the filenames ended with ".en.srt" so Jellyfin correctly associates them with the English language.
I wasn't overly pleased with the seeking either, but I could definitely live with that if subs worked, as the rest was quite nice indeed.