I played crippler 1 wipe ago, I knocked out other people in legs very often, but it was on good critical roll, because it followed with huge damage also. May be you can't KO people, because it's hard when you're shooting arms...? ^_^ Try may be head it's better for KO. ;p
Cripplers are fun, when you're in jacket and deagle crippling hand of that metal guy with avenger and he walks to you unarmed and then you can even take his weapon if he was close to you, the only bad thing is when 5345873465 his friends are after next corner and you see few rockers flying in your direction almost instantly after you picked up his mini.
Also EN matters much, I have 10 EN and no Stonewall perk, because of 1 ST, when they're shooting my eyes and dealing 100-150+ damage with no other effects very often except for bypasses. Also with 10 EN instakills are very rare for you, anyway, if your character build supposed to be hit by others, you better pick 10 EN, of couse it'll be excellent to get 6 ST somehow too and pick Stonewall, and after-wipe all other anti-crit perks +anti-crit trait, so only plasma rifle / .223 pistol eye bypass will stop you by raw damage, if you have less than 10 EN, then you better have 10 PE and something that shoots on ~50 hexes, and you better hide behind your 10 EN friends and then simply shoot those who're trying to pick up mini of your 10 EN friends.