You know thinking about both which is actually the more efficient method of making a shock trooper?
>space marine
>maybe one out of a thousand child candidates even make it to augmentation
>less will even survive to getting the black carapace
>need geneseed to actually do the augmentations which requires harvesting from dead marines and is very scarce
>need constant mental conditioning to do their jobs and stop them from connecting too many dots
>can last for centuries if good enough

>thallax
>can use basically anyone
>subject only needs a will to live in order to pull through the procedure
>just uses existing tech that isn't scarce compared to geneseed
>are in a constant state of pain and dissociation so mental facilities will eventually degrade to the point they are unusable