Edit: Looks like you get 4 concurrent jobs if you register as an open source project.
My biggest complaint has been it lacks an very useful built in ability that Travis had, which is automatically running your builds on the merge of your current branch and target branch. You can right a script to do it yourself, but there missing some environmental variables to do it easily (because the build can start before the target branch is known).
I've also hacked about the 'merge' problem but it's not entirely satisfying.
That all said, with GH actions currently being entirely free, and doing PRs better than CircleCI, we might shift our focus there.