Mordian rules look kinda lame in my opinion, compared to the Catachans. The bonuses to Overwatch are nice, but only worth anything if the enemy actually intends to charge you - and a dedicated assault unit can still charge one squad and then pull in any others nearby through positioning. Which is especially bound to happen to a vehicle that tries to use the rule, given the positioning requirements.
The Mordian-specific order makes up for the lackluster Doctrine somewhat, but won't help them deal with the rank and file...
In short, Mordians will work very well against some enemies (hi there Blood Angels) and not so great against anyone else.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.