Having designed Class-DE amplifiers for RF power applications, I can say the biggest limitation to increasing operating frequency beyond a few MHz is this... The bridge leg's mid-point voltage must commutate from one supply rail to the other during the dead-time. It takes lots of current to charge and discharge the MOSFET's Cds capacitances in a short time. As you increase the operating frequency less time is available for this to take place. So you either have to increase the deadtime to the point where it dramatically eats into the total switching period and considerably reduces output power _OR_ you have to tune the output network to get more circulating current during the deadtime to slew the mid-point voltage quicker. Either of these things actually give you less power out, for a given RMS current through the MOSFETs. As you increase the frequency this issue eventually becomes un-manageable. This is the point where a single transistor topology like Class-E can continue operating up to much higher frequencies, because it is not affected by these problems.