The Doronbo's mechs are usually at advantage against the Yatter mechs until the Surprise Mechas enter the frame (especially egregious the final battle of the original series, where Boyacky's final creation managed to take on all the Yatter mechs at the same time before Gan-chan went for the overkill and gave the Mecanomoto to all of them). How is that the heroes' mechs are weaker than the enemies', even after all the various battles? Gan-chan explained it in the first episode: Yatterwan is a rescue mecha (specifically a fire-fighting one), and the other Yatter mechs, derived from him, are likely the same. It's like putting an armoured fire engine against a tank.
Confirmed in the later sequel series: even when they have the occasion to turn their mech into weapons, the Yatter usually refuse. The Yatter-mech are rescue robots, never fighting tools.
