Capturing @alban's comment:

If the manifest has the following:

      "exec" : [
         "/bin/sh",
         "-c",
         "echo After \nExecStart=/bin/sh -c echo Before\nExecStartPre="
      ],

This gets converted to:

ExecStart="/appexec" "/opt/stage2/busybox/rootfs" "/" "/rkt/env/busybox" "0" "0" "/bin/sh" "-c" "echo After 
ExecStart=/bin/sh -c echo Before
ExecStartPre="

and systemd is complaining about unbalanced quoting and the pod freezes.