mapbox-gl-js focuses on vector tiles and webgl rendering. I'm not sure if it even supports raster tiles, canvas and other "legacy".

Looks like it does support raster tiles: https://www.mapbox.com/mapbox-gl-js/example/map-tiles/

You are right.

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.

It does support raster tiles, using them in production here.

How would you say it is compared to leaflet? much more complex?

We chose it mostly because at the time, it looked like it had more active maintenance from the people at Mapbox, and it eventually looked like it was going to pass Leaflet.

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.