It gets pretty bad as your codebase increases. I work on a pretty large Rust project that uses a lot of async, and uses a LOT of static dispatch (mainly because we use a web-server framework called warp).
Usually, even after a simple change a simple `cargo check` can take a minute or two on a beefy PC. That said, over time you get numbed to it :D.
I can't say much about async because I never really use it. To me it feels like a slightly misleading abstraction/syntax compared to what actually happens, most bugs I introduced in JS backends happened because async/await made the code look more synchronous than it is. Maybe that's just me.
Hey, I'm the maintainer. At the moment Dim is really raw in comparison to plex, there are no clients besides web, and it doesn't display as much data (actors, etc).
I don't know about jellyfin but as a daily Plex user, there are plenty of problems to solve. Not trying to extract money from me by cramming bullshit onto my home screen, not breaking every other update, not failing to stream when using the default "original quality" setting, not showing particular titles on some devices but not others, and not mistaking the Toy Story behind-the-scenes videos for gay pornography would be an improvement over Plex.
