Having everything in one service definitely increases interoperability between those solutions, but it definitely decreases stability. In addition, each of the other systems is not the best in their class (I really detest GH Actions for example).
Why do so many solutions grow so big? Is it done to increase enterprise adoption?
Getting the same level of interoperability with a separate tool takes significantly more work on both sides, so the monolithic approaches tend to thrive because it can get out the door faster and better.
Forgejo is doing the same thing with its actions. Honestly, I'd prefer if something like Woodpecker became the blessed choice instead, and really good integration with diverse tools was the approach.
I do agree there are issues with a single provider for too many components, but I am not sure you get any decreased stability with that verse having a different provider for everything.
That said, I agree that the execution of many features in GitHub has been lacking for some time now. Bugs everywhere and abysmal performance. We're moving to Forgejo at $startup.