For the record, 30k and 40k use the exact same core rules. Moving, shooting, CC, and psychic powers are identical. The legion rules are no more difficult then any other new chapter, it just adds more variety. Primarchs are overrated and could easily be blown apart by Tau and Eldar, although other factions might struggle against them. I understand if tournament organizers don't want to mix the two, but they really aren't that different then any other codex.