You're usually fighting an honorable fight against hordes of forum warriors but in this case you're dead wrong and they happen to be right. WC51 is 100% broken. I doubt you'd find someone who plays the game and disagrees.
And the reasons mentioned for why its that are completely off.
Range and the fact cav rifles can ride it to make it USF M3 version, but superior against all but garrisons.
Health and armor are already normalized to kubel, if we'll get another balance patch, there is a high chance M3 will also be normalised this way.
Arty is completely pointless, because no one ever uses it nor gets it late game for it, mark target I don't know if I've ever seen it used, durability, again, is normalized for kubel already.
Cost? It comes out as worse unit then UC as it isn't even armed and people scream 5-10 fuel map hack is too expensive, how a unit that can exclusively cap wouldn't be?
Certain people with agenda make it sound like its 0cp luchs, when its unarmed M3.