They say that once Millenniummon appears into the field, he cannot be killed (because of his "when destroyed" effect allowing him to revive without cost). Compared to Millenniummon, Ragnaloardmon is far cheaper (3 cost for digivolve, or 0 when you activate his effect), has <Reboot> and <SecurityAttack+1>, but can be destroyed by removal option cards such as [ST1-16] Gaia Force. However, if you can stockpile a bunch of <De-Digivolve-1> effects then he can inject powerful control to the board.