Now there are gallium nitride microwave transistors that can produce very high microwave power at very high efficiency. So that is no longer a limitation.

Microwave ovens with such transistors have been demonstrated, which have the advantage of modulating the microwaves in such a way as to achieve a more uniform heating throughout the oven, than can be achieved with the fixed-frequency magnetrons.

At least for now, such microwave ovens with transistors might be encountered only in some professional applications, because these transistors together with the associated control circuits remain much more expensive than magnetrons.