as i mentioned in the other thread, a brood of carnifexes, will likely kill a knight on the charge.
a D hit only does a d3+1 on average (factoring the d6+6 on a 6 to wound and no effect on a 1) so any mc with 4+ wounds (all the tyranid mcs) will take two D hits on average to kill. it's still not good odds, but with high i and smash, it isn't unreasonable to bring down knights i cc with mcs (though typically not on equal points).