Preclearance in Canada has Global Entry machines so one does not lose that benefit.

They send US immigration and custom officials (and pay them COLA and pay for their housing) to the foreign country so it is quite expensive for the US government to operate, but just a rounding error in its ridiculous Homeland security apparatus.

The downside, especially for economy passengers, is that it means that they have to get to the airport even earlier and connections became more problematic at a hub like FRA. Furthermore, the US customs officials can demand to inspect the connecting passenger's luggage, which becomes problematic for connecting pax.

Imagine the volume of pre-clearance at an airport like FRA with flights to every medium sized US city on LH. I think it only makes sense with O&D airports like Abu Dhabi and Shannon to name some non-Canadian examples. Very few people connect from Europe to the US via Canada.