How does Mapbox stack up against what we've built? Well, they have way more people working on the problem, so there's going to be more support and more eyes on the problem. I didn't directly compare map tiles, that was a couple other people on my team. We decided to try moving forward with our own OSM-based tiles, so the conclusion I've come to is that it compares favorably.

How difficult would it be to build a competitor to Mapbox? Well, if all you care about are map tiles, it's no big deal. If you want to build a business around it, that's always work. You're going to need to build the tiles, have infrastructure to serve it, monitoring, staff to manage it, billing and charging infrastructure, etc...

Mapbox also offers geocoding and navigation, which are quite a bit more difficult than just setting up a server that can host OSM tiles. Obviously, if all you need are maps, that's fine. But if you need maps with search, which I suspect most people do, rolling your own is tougher (though projects like pelias are pretty good).

I set up a pelias system a while ago but it never got any traction. We have another geocoder already set up using Experian QAS data, I think we've had that set up for a decade or more, so that isn't one of our concerns. Good point though.