They are all Level 4 monsters that can be discarded to add their corresponding Field Spell Card from the player's Deck to their hand. While their original ATK ranges from 1600 to 2100, all of them have 2100 ATK when their respective Field Spell is active, except "Neo Space Pathfinder"; Field Searchers with an original ATK of 2100 have an effect that destroy themselves if their corresponding Field Spell Card is not active.
"Neo Space Pathfinder" differs from the other Field Searchers in that it remains at 1800 ATK regardless of the Field Spell Card being or not on the field, it add its corresponding Field Spell Card from the Graveyard to the hand, and it was released outside of Force of the Breaker.
While their effects are little more than specialized versions of "Terraforming", they are all Monster Cards, so they are still useful even when there are none of their corresponding Field Spell Card left in the Deck—"Terraforming", in such a case, would be a dead draw. In addition, all of these cards have outstanding ATK when their respective Field Spell Card is on the field, making them great beatsticks (this is even more true for "Harpie Queen", "Warrior of Atlantis", and "Gravekeeper's Commandant", as the former two already have 1900 ATK, while the latter has one of the highest ATK stats of Level 4 or lower "Gravekeeper's" monsters).
Because most Decks do not rely on more than a single Field Spell card, the fact that they specialize in a specific Field Spell Card does not have a significant drawback.