I play Orks and Space Marines (We typically ignore the who can fight who chart due to so many conflicts of allies in our small group) and whenever I fight an Imperial Knight, even one of them, I have been shot off the table before the end of turn 3. Even the allegedly unstoppable Ork Stompa was destroyed on Turn 1 without any sense of a lucky roll from the Imperial Knight. Every combination of anti-vehicle weaponry I throw at it results in a absolutely crushing loss.
I can't outrun it. It moves 12 inches and still can run after that, or even fire everything at full Ballistic Skill.
I can't out-gun it. Most variations are lined with guns and can take out most units in one to two swift shooting phases.
I can't melee it. It has a high initiative and may simply stomp and slash it's way through even the heaviest infantry, or use it's destroyer weapon table to cut right through the heaviest armor.
I can't meet it with equally tough walkers of my own, the Ork Stompa costs as much as 2 Imperial Knights.
It is so cheap (about 350 points), and riddled with such firepower, I can't take it on with anything
Lets say I make an entire force of anti-vehicle and manage to bring down one of them, there's a second one staring me down right after. Probably with enough points left over to bring in some serious backup to tear up the rest of my army.