I'm quite new to the game, but I noticed very strange thing.
It looks like many heroes skills do not work on Large/Mega units specially if they're not Humanoid.
It started from Cleo's Black Hole, that do not damage or drag mechwalkers and chieftans.
But I see it also on Rufio's Leap, and Mako's shooting star.
On the other hand I see that Avril's blizzard/Ice Nova or Avalons Arrow Voley work fine.
Any clues why it's happening? I didn't see anything in description.
Depending on the size and the Immunity of certain creatures, they might get affected differently by skills. For example, Rock giants are both Massive units and Immune to freeze/poison... anything that is a large unit or bigger will not be affected by most stun mechanics. Rufios leap ability and Makos shooting star might not stun the big targets, but they do the damage... you just can't see that damage on the health bar all the time since its minuscule. From my experience, the best/fastest way to learn about abilities and who is affected is to play the game and learn with time, maybe experiment. The game seems to be very vague about deep in-game mechanics such as these...