The problem isn't that a mirror hits 100%. If you target strong one mirror IS guaranteed to copy the opponent, in fact, two are. The issue is that against a non-mirror involved rit 4 mirrors are GAURENTEED to hit (Again, we're assuming that there are less than 5 of the same critter, we'll call open slots copy critters for easier depiction.)
Issue, if a person uses a six mirror ritual, 3 targetting strong, 3 weak, then 4 WILL hit, unless there are less than 4 crits are in the ritual OR if 5 critters are duplicates. If five are duplicates chances are 4 will be copied anyway.)
Now if the person is using 6 UNIQUE critters the chances of 6 mirrors copying 4 is still guaranteed (With a minimum of 4 mirrors). Chances of copying more than 4 if you are targeting weak with 3 and strong with 3 is 0%. Anywho, it's big and complicated, but what I'm saying is that you can copy a HUGE amount of a persons ritual pretty good certainty using these mirrors. Mirrors + loads o' Ve counter any ritual with 4 or less critters, almost guaranteed. (Though the mirrorer is likely to use 6 critters, so it'd be easier for the mirrored to put more Ve on his useful critters.) Anywho, I am just concerned that number of copies might get out of hand, so as Nex said a limit to the number of mirrors might be a good idea.