I suck with Pythium ... please help!

[Boron]

Hm in theory the hydras are nice .
You could e.g. lead them by a banelord , bane etc. and mix in some undead fodder to prevent routing .
The problem is just that the undead fodder gets killed to easy so it is very costintensive .
You could combine the Hydras with behemoths this should prevent routing and the behemoths should be alive at least sometime .

For Broken Empire ermor they would be much more useful though since they could generate their reanimatefodder undeads in the quantities needed without too high costs

Living statues could work rather well too

Strategic move of 1 and higher upkeep then an Archtheurg limit their usefulness though .

As archaeolept said Principes / Hastatati are just normally more useful .
The "routpreventing" normally takes just too much effort/resources to really consider hydras i think.