The conquerer of 5 states >>>
Riboku even when in defensive mode couldn't secure Zhao's territories and that is when he has all the advantages (biggest is home turf).
Ousen's whole career achievements would be slapping on Riboku's face.
I think Hara won't write clear one sided defeats between Qin 6 and 3GH, it's more like RBK winning the war but the Qin 6 shining more in the actual battle.
Even in previous gen or in current gen, all other states and there titled generals were just securing there lands while it is Qin's 6 which always invades in other states and snatches there lands. Hope you could understand the point. Lol
Qin is the protagonist state which has showed there dream and ambitions so with story point of view, writer can't make protagonist super strong and powerful boss from the beginning but has to build them on that level slowly.
Otherwise historically, few losses aside here and there, Qin was snatching land on almost every campaign. That's there strength and capabilities. Lol