Very often even the layer count is an issue. For perfect example see almost any brand name japanese AV DVD/DVR combo which has giant single layer board with onl

Very often even the layer count is an issue. For perfect example see almost any brand name japanese AV DVD/DVR combo which has giant single layer board with only passives, connectors and links which holds small subboard with bunchs of (fc)BGAs and somewhat incredible layer count. On the other hand for small scale devices not only certification comes into play but also availability and cost. There are niche devices that simply use some Arduino-ish board as a component, because the board is easier to source and, more importantly, cheaper than the MCU on it, personaly I view that as bad design and good reason to use some non-AVR MCU, but that is how it is.