He's talking about using VLC for transcoding or encoding, where the functionality has lots of issues and is kind of bolted on the side. VLC for playing is totally fine.
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.