Ymgarl would be awesome if it was not for the mech heavy meta. The heavy vehicle count means you tend to have to run hiveguard /zoanthropes in every slot.

That being said with the increase in GK players I have been contemplating my one unit of zoanthropes (my list being similar in elite selection as yours with 2 hive guard units and pod of thropes) for a unit of Ymgarl. I reasoned that they lack the inherent problem the thropes have (needing to succeed 4 times before having an effect against a vehicle) and provide some much needed flexibility from the elite slot that those same thropes lack. They can jump long fang squads and vehicles alike the turn they show up and on the charge their 3 str 5 rending attacks should be able to deal with most rear armor.