Honest question: Why would one care about cold or hot spots on the ceiling? I don't spend much time up there.

It’s still a thermal path between inside and outside, and thus wastes energy if you’re trying to maintain an internal temperature that is different from the external temperature. Also there are radiative effects - hot spots on ceiling will radiate IR into your space. Just try standing under a corrugated steel roof in the summer.

Approximately 70% of the heat loss from your body is as radiated heat. Ceiling insulation is critical for thermal comfort. Have a look at the Efficiency Matrix YouTube channel videos on insulation consistency.

The average temperature sure. But would uneven spots have that much effect on the thermal radiation I get sitting over a meter away?

Less that 2% of insulation being out of place can waste something like 40% or some ridiculous amount of energy. It's quite nuts how even just a little break in insulation can have a significantly negative effect. A few years after I moved into my first home - a townhouse - I added some R-16 bats to the already existing R-12 insulation in the attic and it more than cut my heating bill in half. It was a three story narrow, townhouse - not that much square footage of ceiling compared to the walls/rest of the house but it made ALL the difference. If I didn't live it, I wouldn't have believed it. And I kind of did it on a whim since I didn't need that many and Lowes was having a sale on insulation. I would have done it the first week I moved in and had a couple far more comfortable winters!

Okay but that's not what I was asking. That much heat loss will affect the overall temperature.

He did answer what you asked, but I think you may be losing sight of the forest for the trees here. Maybe re-phrasing will help?

They explained how something could cause uneven spots and low average, as far as I can tell. Because those spots leak so much.

But I want to know if there is a way uneven spots could cause a problem without some effect tanking the average. What do uneven spots themselves do?

The uneven spots may indicate the presence of leaks. The leaks can have an effect on the average temperature.

Hot air rises and thus heat escapes through the ceiling if there's spots that aren't well insulated.

Because in the winter the thermal energy within your room will “flow” out of those “cold spots” at surprisingly high rates; in summer those spots act as space heaters. In both scenarios they will adversely affect the ambient temp of the room. In short, whatever method you use to heat/cool the room will have to work harder and will take longer. Here’s [0] a good overview of heat flow in this context, with some hints towards the math behind if you wish to go deeper.

[edit: add link]

[0] https://www.energyvanguard.com/blog/Flat-or-Lumpy-How-Would-...

I might be totally misled, but I think of it as a parallel resister circuit. One area of low resistance is extremely significant.