The output impedance of almost any class D amp is so low that doubling power as impedance is halved is no problem, even into 2 ohms.
The limitations are current capacity in the output section, current capacity in the power supply, heatsink capacity and finally issues surrounding the output filter. Its likely that the latter will be most problematic, which is why self-oscillating is attractive since it will allow the amp to correct for phase shift induced by the filter. Getting output devices that can handle the current is academic at that point- its all about how far you want to push it in the design- you face most of the same constraints you do with a class AB amp.
The limitations are current capacity in the output section, current capacity in the power supply, heatsink capacity and finally issues surrounding the output filter. Its likely that the latter will be most problematic, which is why self-oscillating is attractive since it will allow the amp to correct for phase shift induced by the filter. Getting output devices that can handle the current is academic at that point- its all about how far you want to push it in the design- you face most of the same constraints you do with a class AB amp.