Is Drone useful if you need to build anywhere other than Linux? Yeah Docker can run elsewhere, but it's still Linux.

I'd love an excuse to leave Jenkins for self-hosted multi-platform jobs, but nothing else seems to have the flexibility and to project confidence that it'll still be around and well-maintained in, say, five years.

Nah, Gitlab is the definitve solution.

I really don't feel the need for anything besides gitlab + gitlab-ci.