mapbox-gl-js focuses on vector tiles and webgl rendering. I'm not sure if it even supports raster tiles, canvas and other "legacy".
It came to me, that after all, they have to, if they want to display ortophoto or satellite tiles.
I was thinking more in the terms of street maps or thematic maps, where mapbox-gl-js displays them in a very nice way using vector tiles.
I wouldn't say it's more complex. mapbox-gl favors a more declarative style (look into "Map Style" in the docs), but the concepts are not that different than Leaflet.
Web GL features (such as video support) is an obvious advantage, and part of why we chose it. We don't use plugins for this project, so can't say much about that.