Because there is nothing can touch the sleep mode, so this deck have a very minor disadvantage against deck that bound back to hand like MirageGao (you can't trigger On Deletion effects). The LV5 Astamon and Gizumon XT able to proceed with the strategy in the same turn they are played, not stay till next turn to get it bounce back.
The other problem is we cannot draw the Sleep Mode and have to move on with the vulnerable Rage Mode, lolx, or opponent play the blocker (Royal Knight). The Tamer Kurata's "End of Opponent's turn" effect can only destroy the opponent's LV6 Digimon, it can't do anything with Royal Knight 's Craniamon, while other blockers are not lv6 Digimon (mostly).
But in early of the game, if we are able to draw many Gizumon* in hand then the game will run very fast, we may be able to finish opponent's security before they can set up the defence against this deck.
And if you are reading this article, do check out the other version of Gizumon + Anubismon Rush, there are some tournament winning decks of this combo in BT13 decklist.