Diesel engines run at 2-3Bar (absolute) manifold pressure at sea level, yr right. They need all this air because even at full power, they are running lean. The better the combustion system, the closer they can run to stoichiometric before they reach their smoke limit - i.e. how rich before soot emissions become unacceptable.
If you don't have any boost at all, they will have half or less of their power, even at sea level, therefore the critical altitude of a heavily boosted diesel engine - which is most of them - is sea level.
Highly boosted diesel engines usually run lower compression ratios due to structural limits for the maximum cylinder pressure. Low compression ratios means low peak cylinder temperatures under compression which limits the starting capability. That's why diesel engines run glow plugs and inlet air heaters for starting assistance - these are usually only man enough to start a stationary engine - not one that has it's cooling system running flat out, as it is for air cooled engines in flight. Lower atmospheric pressure at altitude also reduces peak compression temperatures too, with similar results. Combine the two and your diesel engine might be very hard-to-impossible to restart at altitude.
Even low power levels below the turbocharger boost threshold can be enough to let the fire go out. I believe this is why the SMA engines have a minimum power level for descent.