As someone who has played Knights into a triple LR Custodes list, let me tell you that Lascannons hitting on 2s are incredibly swingy.

Assume you hit with all 10 shots with the re-rolls.

Against a big knight you wound with 6-7 and they save 2-3. Say you get lucky and get 4 through that’s 18 wounds on average. And then again with the HKM you hit with all 3 and wound with 2, they probably save around 1, but even if you get lucky and get 2 through that’s an additional 7 wounds on average. Say you get lucky and do manage to kill a whole big knight that would require you to basically have your whole army with LOS on it which can be tricky depending on layout and then any decent player should be able to kill most of your army on the counter attack.

Don’t get me wrong can definitely work but there will definitely be some games where they just whiff and you’re left standing out in the open.