I migrated from BitBucket recently and was surprised at how BitBucket was often ahead of GitHub in CI and API breadth. I guess I mistakenly thought GitHub would be miles ahead with its popularity and backing.
Their pipelines was woefully underbaked and had tons of issues last I used it which was about 2 years ago. Pipelines was incredibly slow compared to Actions and was missing most of the features Actions has. You can't even define scheduled jobs from within the YAML file. You need to schedule them outside through the repository settings. It's also missing tons of triggers and the documentation is horrendous
I've not used their APIs but GitHub's is pretty huge. I highly doubt BitBucket's API is any better