One difference I can immediately point to is that VLC always renders subtitles at the video's storage resolution and then up/downscales all bitmaps returned by libass individually before blending them. This can create ugly ringing artifacts on text.

I've also seen many reports of it lagging or choking on complex subtitles, though I haven't had the time to investigate that myself yet.

Either way, it's not as simple as "both players use libass." Libass handles the rasterization and layout of subtitles, but players need to handle the color space mangling and blending, and there can be big differences there.