Below you'll find some of the details of how things changed between the new jail utilities and the old ones.
It's possible to start two jails with the same host name. Not necessarily a good idea, but possible. The old jails couldn't distinguish between the processes running in two jails with the same host name, and would only print the name once. The new jails command lists it twice as expected.
These are completely compatible drop in replacements for each other. In fact the new utilities makes a hard link from killjail to jkill so you can continue to use it as expected. When executing shutdown or startup scripts the old killjail would signal jailer which would run the scripts. jkill now runs them itself without need for a jailer process running in the jail.
jstart cleans up the environment and does some other security conscious things before starting a jail. If you relied on passing environment variables into your jail, or other things outlined in the man page you probably want to stick with using the jail command.