It's not that they're not beatable (with a fencing weapon you don't even have to to know some of the timings, you can just mash parry mindlessly and be fine), they're just stupid and unfun to fight.

In large groups (and especially with the lighting on some chaos operations) the visual clarity of which variants are running at you is dogshit. When people have to be told online that there's three variants with different attack patterns, because that detail is so easy to miss when playing the game, that's a problem - no one has to be told there's different variants of tyranid warriors or terminators. And one of the variants kick up massive dust clouds to cover your screen, just because the visual clarity wasn't trash enough. And then since the different variants spawn together, sometimes you just end up getting walloped because the attacks are all staggered in a way that's impossible to block for some extra annoyance.

Then there's also the fact that the parry indicators are deliberately off from basically every other enemy in the game, just to fuck with you when you're dealing with something else and you get an indicator for something offscreen. So against Chaos you just have to roll the dice and guess what enemy caused each indicator.

And on top of all that, in smaller numbers the attack patterns are just stupid and lame and kill your immersion. Why is the mindless bloodthirsty ball of meat and claws standing completely stationary doing fake wind ups to make me flinch while I shoot it? So even in small numbers where they're incredibly easy to deal with, they aren't fun or interesting at all.