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The title says it all; I'm looking for any qualitative information that can help me.
Freezers: https://www.energystar.gov/productfinder/product/certified-residential-freezers/details/2255167 Refrigerators: https://www.energystar.gov/most-efficient/me-certified-refrigerators/results
Per this question, the testing process for refrigerators costs money.
Although it seems logical that a refrigerator would be more efficient since it doesn't have to keep things as cold, there are a few different factors affecting real-world performance.
Freezers are simpler to operate. In a freezer there's only a top end on the temperature range. From refrigerator manufacturer LG you can see the range in the refrigerator is narrow, but the freezer range is broad:
For a freezer, the compressor can run for a while to get the compartment well below zero. During start-up and stopping, the compressor is less efficient, so it makes sense to run it for a while.
But in a refrigerator, if you run the compressor too long, you risk freezing things that shouldn't be frozen.
Fridges need a "relief" valve. If a fridge does get too cold, it needs to be able to warm back up again. I suspect this is why the insulation is thinner in the fridge compartment -- if it's too thick and the compartment gets too cold, there's no way to warm it back up. But having thinner insulation means that if the compartment gets a bit too cold, it will naturally warm back up a bit. This allows the thermostat and control system to be simpler (and cheaper).
Freezers aren't opened as often. This part gets to the real-world aspect of Energy Star ratings. It's 11am where I am, and so far I've opened my refrigerator three times: creamer for my coffee, orange juice with breakfast, and an apple for a snack. I've only opened my freezer once to get some frozen fruit for my oatmeal. So the testing criteria likely emulates this real-world usage.
