Guilliman and the time-locked stasis seal...

[Cap'nSmurfs]

Sure. And the whole system of tithing, control and observation is precisely because of the too-fast and unobserved recruiting practices of the Heresy, all of which went very badly wrong. Too many instabilities went unnoticed or ignored. It's all but outright stated that many of the Traitor Legions had flaws in their gene-seed, their recruitment practices, or both. Some of these flaws, like the Word Bearers', were very subtle, and need not even necessarily have been flaws if events had taken another turn.

This has always been the reason Ultramarines gene-stock has been the preferred source of new Chapters. It's stable, it doesn't mutate, it doesn't have crazy quirks. It's reliable. Even the Imperial Fists, who I believe are the second most commonly used and are almost as reliable on a genetic level, have a couple of mutations which mean a couple of the Space Marine organs don't work properly, and there's the tendency towards producing marines who are self-denying to the point of throwing themselves into fights they can't possibly win. Which isn't necessarily a bad thing in the 40k universe, but it isn't always what you need to be able to count on.

I think it's a Captain Agemman line in one of the Graham McNeill Ultramarines books which points out that the point of the Codex Astartes and reorganization is not to make perfect Space Marines but LOYAL Space Marines. It's what all the reforms are for; making sure something like the Heresy could never happen again.

[Katharon]

...the Codex Astartes and reorganization is not to make perfect Space Marines but LOYAL Space Marines. It's what all the reforms are for; making sure something like the Heresy could never happen again.

Not so much loyal space marines, but that no one man or space marine lord would ever have in their power that which the Primarchs did in controlling legions. But yeah, I get what you mean.