Its not like massing Paladins, they cost more, take up more population, and probabaly take a while to create (well..in this case, summon)
They aren't quite invincible, as said before, you'll be lucky to mass 20 of them, at the most 50. You have your GPs to use against masses of them, and if someone turtles, you also have time to turtle along and mass troops and MUs.
If someone is turtling building MUs, make human units and a few MUs and atack them when they don't have tons of them yet..since they probably take a while to summon like I said before.
DK