Kanbei is a broken-tier CO whose units are more expensive and much stronger than regular units. As a result, he does not appear often in regular play. Rather, he has situational use in casual play and catered competitions.
Day-to-day Effects: All of Kanbei's units have 30% increased offense and defense, but cost 20% more to deploy.
Power: Morale Boost -- All of Kanbei's units gain 20% increased offense and 10% increased defense.
Super: Samurai Spirit -- All of Kanbei's units gain 20% increased offense and 30% increased defense. Kanbei's units also have an additional 50% increased offense when counterattacking.
Kanbei's early game is subject to a vehicle count disadvantage due to increased costs. Therefore, it is better to focus on nearby properties to gain a funding advantage. Contested properties will be easy to recover later on.
You may have to build a lot more infantry, and a lot fewer tanks, given the limits of funding. However, the strength of your vehicles will compensate for the lower vehicle count. Units of most COs will actually take more damage than they deal in same-unit matchups, even when they get first strike.
Expect opponents to tech-up sooner to overpower your tanks. This means more frequent MD tanks in play. Conservative use of Tanks and Artillery will be sufficient counters.
Use Samurai Spirit (SCOP) over Morale Boost (COP) in most situations. Morale Boost is rather weak for 4 stars, while the added defense and counterattack potential of Samurai Spirit will make one's units invincible on 4-star terrain and near-invincible on all others.
Kanbei may have issues on maps where he has to build transports; building them will take a large cut from the funds he needs early on. Reconsider before picking Kanbei on such maps.
Kanbei's broken nature comes from his increased defensive power, which is key in AWBW. Players can expect attacks to be countered with equal or greater force from Kanbei's (e.g. Tank v. Tank). Because of this, opponents must play with great caution. This gives Kanbei the chance to advance more freely.
The second point to this is Kanbei's SCOP, which gives him an essentially free turn. The increase in defense makes opposing attacks relatively useless and the threat of counter dissuades any direct action. During the SCOP, It is especially dangerous when an invincible infantry caps an HQ. As such, the SCOP can function as a decisive game-winning move.