If there's more than one consumer unit, powerline adapters may not work. So the garage might have a lot to do with things if there's interesting electrics. But your main point is not correct as virtually everyone has a single feed to the house - it's what happens after the main CU that is important.
I have a hybrid with Eero covering the main mesh network for the house, but have a wired backhaul, with one Eero attached to a powerline adapter for one room, on the same consumer unit. Generally Eero mesh is fantastic though. But you cannot fight against physics - thick walls are thick walls.
