might be a little disadvantage because you have to move your mouse to the picture - hotkeys requieres no moving :O
I think clicking on picture is way faster if you have more military production buildings, most commonly in CA wars, you can switch archeries rly fast and just click production button with other hand using mouse without wasting time to move your fingers on keyboard especially if you have to use ctrl+ to go to building.
Also it takes like 0.0001 secs to move your cursor to the unit button location, and even if you use only hotkeys, no way u re doing anything useful with ur mouse while switching production buildings..