Switch from sauce-connect-launcher to sauce-tunnel.
sauce-connect-launcher never worked on my machine (logs didn't shed any light), though apparently worked on Drone.io;
sauce-tunnel works for me.
Still, it's flaky on public wifis, so I suspect sc itself has NAT/ports trouble and the difference was just me sampling on different networks?

A downside of sauce-tunnel is that its git repo includes the sc binaries, which I'm not sure they have a right to.
[smoke test] Run a static server on localhost:8000 and try to access it from Sauce servers.
Works in a local browser but Sauce servers say I'm not using a tunnel :-?
Probably need to include the tunnel id in the Sauce browser capabilities.
Log incoming requests to static file server.