The QCX is a very different beast from the uBitx.
The QCX uses a class E final the expects the transistor to act very much like a switch.
The advantage is extremely high efficiency over 85% its typical or higher and very
little power expended in the device. The need is the device must switch fast and hard
and IRF510 and RD parts while they can do this need much more energy into the
gate to do it. The reward is if done well a single IRF510 will do 30-40W at 12V, however
its not a drop in thing. These are typically monoband designs. This works well for
CW, AM, and other constant carrier modes.
The uBitx uses a series of linear amp stages that include the finals. At best efficiency of
each stage may be 55% or lower with a fair amount of power and heat expended.
This is a requirement for undistorted SSB and multiband designs.
What applies to QCX and uBitx are very different.