Found a case where ey ssh worked but ey deploy was failing the SSH authentication. In the end, the problem was solved by fixing a problem with the SSH config.

It would have helped if the ey deploy verbose logging included the SSH authentication debug logs. SSH config was already configured to show debug logs, but these are not displayed by ey deploy. Or, perhaps the debug logs can be displayed when supplied with a very verbose option (vv, currently unsupported).