My issue (with the Cardiff routes, and even more so with the Cambridge routes) is whether they can make economic sense for the airline. Personally I don't think they can.
As I am sure you are aware, in the short term economic sense is not profitability. Cityjet has surplus capacity since cutting back at London City. Therefore in my view, as long as these routes cover their direct operating costs, it makes economic sense for Cityjet to operate them for the time being.
I wish Cityjet the best for this new venture. Cambridge Airport has done well to attract an appropriate operator to fill the void left by Darwin so quickly.