Render advertises zero downtime however if you need to deploy several services it's challenging to minimise the time they are out of sync for.

Building one service may take significantly longer than building another.

For this reason it would be nice to be able to prep a built-version of a service and then, in a separate command, to be able to switch traffic to the new service. That would allow for much better synchronisation in the release of multiple services (e.g. a back end and a front end).

Not only that but it would allow the build step to happen in parallel with testing which would shave another minute or so off of the CI cycle. Because a deploy has to happen in lockstep with a build, neither of them can be started until tests have passed.

If it were possible to decouple the two then the build step could run in parallel with the tests. Deployment would be the final, instantaneous command. Heroku allows for this with build slugs that can then be rolled directly into production.