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
This was pretty much a curve-filler in Limited, although there are some Defender themed decks that can always use the ability to attack every now and then that wont mind another option
Also, if they get another role, they'll die so that'll shut down those from getting played
Throwing away random Treasure or even synergizing with Experimental Synthesizer and its ilk makes this card filtration and a synergy piece
The cards are undoubtedly powerful, but are they actually doing anything to make for a fun game
Cards like Impact Tremors and Purphoros, God of the Forge are great picks here