All the Crop Harvesters have the same job and do it equally well, but you will need to find more as you find and unlock other islands. There are a few important things you should be aware of.
Depending on the friendship level you have with them will affect the amount of crops they harvest in a day. This can range anywhere from 6 items a day to the entire island. That means it is very important to brush these monsters daily to get the most out of them. If the storage for the island is full, they will also stop harvesting, so empty it frequently.
The do not replant the crops in any way. Once the crop is picked up, it will be open for weeds and grasses to grow. These will be harvested too, but you will need to return to plant new crops.
There are also a few things that they do not harvest. Trees and Rune Crystals that are randomly left behind when they mature are completely ignored. That means it is your job to return to the island to collect these Crystals and cut the trees. Also do not put any on Winter Island. They don't do anything there but eat your cookies. Harvesting Ore is just another thing that you must do on your own.
D-2 Shrine, G-2 - Frost Island, H-4 - Mystic Ruins, H-4 - Wrecked Ship
Wind Spirit Shrine 2F, Wind Spirit Shrine 3F
C-5 Ancient Statue, F-9 Nameless Isle, Flame Spirit Shrine 1F, Flame Spirit Shrine 2F, G-7 Ancient Statue
Wind Spirit Shrine 1F, Wind Spirit Shrine 2F
C-3 Shrine, F-6 Wrecked Ship
G-6 Statue Island, Water Spirit Shrine 1F
Earth Spirit Shrine 2F, Earth Spirit Shrine 3F, D-3 Shrine, F-6 Wrecked Ship, E-4 Wrecked Ship
C-3 Shipwreck Isle, D-5 Nameless Isle, Earth Spirit Shrine 1F, Earth Spirit Shrine 2F, Earth Spirit Shrine 3F
D-2 Shrine, G-3 Ancient Ruins, Water Spirit Shrine 1F, Water Spirit Shrine 2F