Temple of Malice isnt legal and you have early-drops that punish you for having lands enter the battlefield tapped, and this deck is very mana-intensive in general
In the manga[] In the original and R manga, the Fusion Material Monsters must exist on the field to be fused with "Polymerization", and once they are fused, they are still treated as separate cards for the purpose of specific card effects (most famously, "The Wicked Eraser", which treated the 3-Material "Arcana Knight Joker" as 3 cards and gained 3000 ATK)
If it's a Siege, it will still be exiled when the last defense counter is removed from it, and its controller may still cast it transformed without paying its mana cost
Typically, having very few cards in your library is a nightmare scenario, so cards like Doomsday or Leveler are trying to enact a harsh penalty that turns out to be a boon
Be careful with the first effect of Red-Eyes Archfiend of Lightning as you can use it to bring out powerful Fusion monsters