If Fecund Greenshell's second ability stops applying (because Fecund Greenshell leaves the battlefield or loses its abilities or because you no longer control ten or more lands), then any creatures that needed the toughness bonus to stay alive will die
The loss of Arven and Technical Machine: Turbo Energize makes it harder to charge attackers quickly and find Precious Trolley in the early game
That not only makes them easier to understand for newcomers, but it also means that if you have fun with, say, the White Calling All Angels deck, you can start swapping in cards of that color to customize it as you see fit
Dredge decks are the best homes for cards like Dread Return
His second ability benefits further from this, growing Firkraag and drawing a card everything a goaded creature deals combat damage
Jump-start is just the cherry on top here, as you can copy that creature again easily, or hold out to copy a better creature later on