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.