Psienesis wrote:The Custodes are designed as solo-operatives, bodyguards and assassins. They are not designed for, and are actually not very good at, military-style operations or tactics. They are not soldiers, they are assassins.
The Primarchs are superior because they are created wholly from the Emperor's science project to develop gene-seed. Unlike Space Marines, which are regular kids who are implanted with gene-seed and the various upgrades, the Primarchs were built from the ground up.
Almost. The Custodes were guards and assassins. Though that combination does seem odd, it makes sense - "send a thief to catch a thief" and all that.
They could perform well in a battle due to their size and strength, but due to their cost of production it's more efficient to send something relatively expendable instead - like a Space Marine.
The Primarchs were the perfect leaders - They were strong, tough, highly charismatic and even more intelligent.
In short:
Primarchs were created to lead
Custodes were created to protect
Thunder Warriors were created to fight and kill
Space Marines were created to fight and obey