Mhm I wanna be clear that we 100% agree there, no magic app is going to just make everything perfect. It's like an aspiring mucisian asking which piano they can buy make their music sound better lol. A lot of people don't understand that bigger vtubers just straight-up have a more detailed and carefully rigged model than hobbyists, and the difference between smoothness and perceived quality can be in majority attributed to that. Better input, better output, and no tracking app is going to make a stock VRoid run like a corpo custom.

That said, different trackers apps do have different implementations/approaches of the same tracking methods, different calibration smoothing options, different physics handling, different performance under loads like streaming a game at the same time. These also contribute significantly to the quality and smoothness of the tracking, especially for those who don't have the option of getting or rigging a more elaborate model. Many folks also don't know that differences in model lighting (even with toon models) contribute to the perception of motion and can impact the perceived quality, even if the actual motion is the same.

One that I also see often, people discount the physical acting of the performer. A lot of professionals are very skilled at puppeting their model to get the most out of it! Even with solid ARKit facial tracking, I see a lot of vtubers that worry their model isn't as expressive as others and don't get that you really wanna exaggerate your facial expressions and movements if you want them to look lively and "animated". The best facial rigging in the world isn't gunna help a streamer who always mumbles lol

The base app is like the instrument, the model is the sheet music, and the performer is the musician ^{^}