Like most Magic: The Gathering cards, the value of Those Who Serve depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Amonkhet)
I think of tcg as holding onto the past, reliving it a bit, but not a massive new territory that is unexplored with endless growth
Not only can you hold up countermagic to freeze the boardstate, but you can sink excess mana into scry effects, immediately increasing the power of your board rather than needing to first spend a turn drawing cards, then casting them and then eventually being able to attack
Creatures that Sacrifice Themselves Sakura-Tribe Elder and Selfless Spirit create a lot of value: with Brenard on board, they fulfill their slay role one again, or more, if a token doubler is on the board
Lilah, Undefeated Slickshot will be a neat commander, but its clearly at its best with Kalamax, the Stormsire or Kess, Dissident Mage