Is there some rule that says if a person is playing on console in lethal they must select a melta capable class or tactical with grenade launcher? 

Just kidding, but it seems that way.  I play on pc and have crossplay on.  The past 8 people on xbox/ps that I have come across were using either a melta or grenade launcher.  As soon as they were in a situation where they couldn't rely on the weapon they get dropped.  The one console player on lethal that came in as a bulwark on the other hand was phenomenal. Team heal on terminus executions, wade into huge groups and barely get scratched.  Saved the op by lasting almost whole death timer against many chaos enemies to let us get back into the fight.  Hats off to that dude.