It's all about the bonus to damage in the rear of the tank, which harasser does not have. If there was just a weaker armor - it wouldn't be a problem, but it looks like there is a damage multiplier. In any case, one Viper magazine is enough to make lightning start to burn if fired at the stern, although the damage amount is only 1,500 out of 4,000 health. That is, harasser de facto has more powerful armor than tanks. At the same time, it is damaged by light infantry weapons. To quote major Payne: "- I don't feel logic! "That's because she's not here!""