The main difference between Parasol Zombie and Umbrella Leaf is that Zombies cannot have more than 1 zombie on a lane, that's why Parasol is an Untrickable Zombie itself.
The other reason is that zombies can get more value out of their "board clears" because of the amount of targets that can be hit with a single trick.
When a Plant player uses an AoE attack/trick, you can hit 3 zombies MAX.
When a Zombie player uses an AoE trick, you can EASILY hit 3 plants. (Again, because plants have the Team-Up plants).
I believe that because Umbrella Leaf can protect up to 5 plants and from game-changing tricks (like Weed Spray) it is fair that it is not an Untrickable plant itself.
Another small thing, Guardian is getting the 9-health untrickable nut next expansion, so the amount of Untrickable things will be even by then. (A untrickable single minion and a minion that gives untrickable for both sides).
