We're looking into this. Third generation servers seems to have two ways of running initial scripts, just like first generation servers, the alternative way generally seems more stable than cloud-init. Meanwhile we're also looking into adding a
script parameter which takes an entire script instead of just a script URL, this script will then be stored in our database for convenience.
In the end, we'll be able to offer multiple options of running installation scripts during first boot and the usage should be the same on all generations of servers.