Do you mind to elaborate? We have migrated from Travis to Github Actions and love it so far.
I haven't used GitLab for a while because most of the projects I'm working on are on GitHub, so I can't comment on GitLab CI.
The most common case where this matters is if someone pushes a failing commit to master, and then all PRs based on that commit will also fail, and then a fix is pushed to master. With Travis, you just need to re-run checks, but with GitHub actions, all those PRs need to be rebased on the new master.
Actually, actions/checkout will check out the merged tree by default.