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To your title, not very. You add in a huge amount of wasted space so that the thing can operate in either mode (and keep in mind miniturization can only take you so far), in the end you will have a vehicle that it at best moderatly good in both modes but would get its ass handed to it if it ran into a unit designed specifically for either one. They are really just the next step-up in the 'humanoid robots/armor look cool' anime trend.