It seems Cirrus CI got slipped under the radar but it's worth to mention because it perfectly handles all the critical features and has much more: https://cirrus-ci.org/

1. Cirrus CI scales from 0. With Cirrus CI you can either use infrastructure managed by us and pay only for CPU hours used for CI without a limit on how many parallels builds you run. Or you can connect your own infrastructure for the hybrid approach and still be able to scale to 0 when there is no load.

2. Great support for monorepos. From ability of using up to 96 cores for builds to skipping parts of the "pipeline" based on changed files.

3. Cirrus CI is not as popular as the finalists of the blog post but still somewhat popular and we are actively growing/innovating.