Their trigger reads, Whenever a land enters the battlefield under your control, you may gain control of target creature for as long as you control Roil Elemental. So, basically, as long as you can keep Roil Elemental alive, you can absolutely steal every other Creature on the battlefield
To help navigate that choice, consider three factors: Your land count : If youre light on mana, chances are your hand is full of spells, so playing the card for its normal cost may help you influence the board more efficiently
Sorcery-speed cards make all control players cry, especially if they come from the reactive school of control
Check it out: You can find this version of Sothera, the Supervoid only in Edge of Eternities Collector Boosters
Sources: PriceCharting (2), TCGPlayer (2), Ptcgradio/YouTube
As long as Frodo is your Ring-bearer, it must be blocked if able