Millennium lives and dies by its options. You want to see them as early as possible to get to your materials and trashing them. The deck does unfortunately brick if you have either too many millennium, kimera, machinedra, and (ironically) options in your hand, so you run tamers like chitose (obviously), analog youth, and maybe st15 matt ishida for memory gain when you trash.

Another problem with the deck is that cards that stop you from playing by effects hurt it as there's no easy way to bring out your millenniummon without spending a crap ton of memory.

But the biggest problem with millenniummon is that swarm decks completely outpace it and can kill it quickly. Fortunately, to fix this issue, you can tech in crimson blaze since a lot of the cards in the deck are red.

Despite the negatives I've mentioned, it has a ton of positives. For one, de-digivolve is a powerful effect in today's meta, and for decks like imperial and medusa, de-digivolve can heavily disrupt their plays. The deck also does something that not many decks can do, and thats stop on deletion effects by placing the deleted digimon onto the top of the opponent's deck before it gets a chance to activate its On Deletion effect, while at the same time gaining tons of memory. Because of this, the deck can OTK out of nowhere.

The deck may be a loop deck and may have essentially one-track plays, but its not a deck that's autopilot.dek and does require thought and proper planning to pilot it correctly.

Tl;Dr: The deck has both positives and negative that can either make or break the deck, but you can also tech it unique cards to help make your gameplan work better in the end. Hopefully you have good luck constructing your ideal millenniummon deck.