It's also a massive PITA trying to debug or profiling performance on it - no native tools work right.
While "ok" for porting an existing game as a secondary platform, you don't want to be using it for primary development. And that will be why it will remain (at best) a second class citizen.
Yes, I want to be very clear GP was in response to the argument "Valve can't port games to macOS because macOS doesn't support cross-platform graphics APIs". Well, no, there is a compatibility layer, performance isn't stellar but if the alternative is your game doesn't run at all, you can take the hit.
Apple should not be happy about this outcome. It doesn't help them build some kind of walled garden, it just makes their hardware perform worse. They should add Vulkan support!
>there is a compatibility layer, performance isn't stellar but if the alternative is your game doesn't run at all, you can take the hit
They don't make many games these days, but Valve tends to be a studio that prides itself on performance. It can very much be "this game runs well or we don't use that platform at all" for them, compared to other AAA studios happy to take the PR hit for a quick buck.
While "ok" for porting an existing game as a secondary platform, you don't want to be using it for primary development. And that will be why it will remain (at best) a second class citizen.