In the title sequence, Chani tells us "At night the spice harvesters land". We see some of them: they look like giant insects.
Later, after House Atreides takes over management of Arrakis from House Harkonnen we follow them on a tour of spice harvesting operations. We see more conventional machines in use, with large crawler treads.
We're given to understand that the Atreides are taking over spice production as a going concern, which presumably includes the existing plant and machinery. And in any case they will not have had time to develop and field-test new spice harvesters. So why do we not see Atreides using the same equipment as Harkonnen?