My argument with split systems, at least how they are installed in NZ, is that they are usually installed in the main living area and if there is a second unit, in a corridor.
So corridors are heated or cooled far hotter or colder than they need to be in order to heat/cool bedrooms.
One has mini splits. You definitely don’t generally put mini splits in every single room (like bathrooms or interior hallways) because each mini split requires tubing, is large, and expensive. No, I think it’s way more likely that you would have them in the bedrooms rather than the hallway
But we have many very toasty hallways leading to cold single glazed bedrooms!
In the last few years the central govt has brought in legislation requiring landlords to heat and insulate homes to a certain standard.
They also recently undid a long held myth about ducts buried in insulation leading to condensation issues. Most areas now allow ducts to be buried as long as you have R19 above and below, or R30 above. When I just installed a ducted mini for 3 upstairs bedrooms, I surrounded my R8 flex duct with R30 insulation.
Like you said, it’s physics.
Most roofs in Australia still aren’t sealed. The air barrier and insulation barrier is the ceiling. The roof space itself is not insulated, so the ducts are exposed to extreme temperatures, thus destroying the efficiency.
Here is just one article on it: https://newenergythinking.com/2018/10/20/dont-use-ducts/