Most decks in the game will not go above 40 cards to help with consistency, but some play styles, like the Lightsworn archetype or decks used That Grass Looks Greener before it was banned, can benefit from more cards
Dedicated myriad builds can flood the board and overwhelm opponents before they can stabilize
I think there are various ways to get energy counters, some of them harder than others and thats a problem
[Your Turn][Once Per Turn] When a card is removed from your opponent's security stack, delete 1 of your opponents Digimon with DP less than or equal to this Digimons DP
Just use one of 50 different ways to give it Trample, or some form of evasion, and that's game
Heroic Intervention and Plaza of Heroes are powerful protection spells, and the former is one of the most played protection spells for Commander