The first major group of Field Searchers were released in Force of the Breaker. All of these Field Searchers are Level 4 and have 2100 ATK while their corresponding Field Spell is active; the monsters with 2100 original ATK also destroy themselves if they are face-up on the field but their Field Spell is not active. In Force of the Breaker, all of the Field Searchers were Rare, except "Elemental HERO Captain Gold" which was Ultra Rare.
Field Searchers first printed after Force of the Breaker no longer have these properties in common. They may have other effects in addition to searching the Field Spell.
Force of the Breaker
Post-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.