To be fair, driver vendors have gotten a lot better. And the spec itself has matured tremendously, with some great features that don't even have direct equivalents in Direct3D.

Sadly as a whole the API still sucks, especially if you care about supporting the vast majority of the audience who want to play video games. Issues I've hit in particular:

Every vendor's shader compiler is broken in different ways. You HAVE to test all your shaders on each vendor, if not on each OS/vendor combination or even each OS/vendor/common driver combination. Many people are running outdated drivers or have some sort of crazy GPU-switching solution.

The API is still full of performance landmines, with a half-dozen ways to accomplish various goals and no consistent fast-path across all the vendors.

At least on Windows, OpenGL debugging is a horror show, especially if you compare it with the robust DirectX debugging tools (PIX, etc) or the debugging tools on consoles.

Threading is basically a non-starter. You can get it to work on certain configurations but doing threading with OpenGL across a wide variety of machines is REALLY HARD, to the point that sometimes driver vendors will tell you themselves that you shouldn't bother. In most cases the extent of threading I see in shipped games is using a thread to load textures in the background (this is relatively well-supported, though I've still seen it cause issues on user machines.)

OpenGL's documentation is still spotty in places and some of the default behaviors are bizarre. The most egregious example I can think of is texture completeness; texture completeness is a baffling design decision in the spec that results in your textures mysteriously sampling as pure black. Texture completeness is not something you will find mentioned or described anywhere in documentation; the only way to find out about it is to read over the entire OpenGL spec, because they shoved it in an area you wouldn't be looking to debug texturing/shading issues. I personally tend to lose a day to this every project or two, and I know other experienced developers who still get caught by it.

I should follow with the caveat that Valve claims GL is faster than Direct3D, and I don't doubt that for their use cases it is. In practice I've never had my OpenGL backend outperform my Direct3D backend on user machines, in part because I can exploit threads on D3D and I can't on GL.