Don't have any particular OpenLayers examples in mind at the moment but it's very heavily used in government, enterprise and academics — sectors that often need advanced projections, use various obscure formats / protocols and prefer all the features to be packed in one product and maintained consistently by one team.

For Mapbox, here's a showcase https://www.mapbox.com/showcase. It especially shines in apps that need to render hundreds of thousands of features at once, maximum customization in how the map looks and feels, updating map appearance dynamically in real time, or some cool animations.

In terms of features, Leaflet has a huge plugin community and there's a plugin for pretty much any feature since it has been around for 8 years, although their quality varies (the core is top-notch though). Mapbox GL will need to catch up on that front certainly, but on the other hand, there are things that are simply not possible with raster-based mapping libraries — things like making any feature you see interactive, changing the map appearance in real time, rendering millions of features (with high perf viz features like clustering, heatmaps, advanced labeling etc.), and general smoothness of user experience (smooth zoom transitions, labels adjusting to rotation and pitch, etc.).

There are mapbox-gl-draw plugins to make it comparable to Leaflet.draw but these are features IMO that mapbox-gl-draw probably should have had in the first place.

So, as someone who works with mapbox-gl-js a lot, that is something I see as a gap.

When I was working with this stuff, Leaflet was more "view only" and not so much editing. This was back in the "have to support IE6" days. I remember trying Leaflet for a simple project, but gave up since I could not set a bounding box for panning (the map was only supposed to show a limited area of the world), while OpenLayers seemed had every API feature one could wish. Although more bloated, less polished :)

I've used Leaflet for a few years and now OpenLayers for about 5 years to author UIs for monitoring robot fleets in real-time as well as build Web-GIS applications for mapping and configuring, autogenerating, validating the spatial datasets (fiducials, SLAM data, navigable positions, "zones", "pathways", etc.)

Leaflet is what I'd say will fit most people's needs when it comes to your standard web-map use cases. It's a beautifully designed library with a good level of modularity and lots of plugins. The API is great.

OpenLayers shines when you get into a lot more complex stuff. The symbology engine is phenomenally powerful and I found that the API plugs into more complex Redux-style state management in a more easy way than Leaflet.

I also found performance when doing high-volume, high-frequency data updates and interactions to be faster with OpenLayers.

That being said, I haven't dug deeply into Leaflet in about a year and things can change quite quickly!

I suspect the overwhelming majority of developers aren't doing anything all that novel or complicated. They're not making Web-GIS applications and probably will do just fine with either.

EDIT: My favorite open source library right now is maptalks. Basically mapbox sort of functionality and rendering, but totally free.

We were going to do something to optimize it in OpenLayers by simplifying the shapes, but when we tried it in Leaflet it just worked a lot faster (fast redraws). We think it's something around how Leaflet dynamically switches to a canvas renderer for big datasets.

Although the image is not a map at all, I tiled it and served it as a custom map layer. You can view it at https://finance.alisio-computing.eu