So, here's the thing, Harass with Tau is really ideal, see for yourself – they quickly move into those... Whispers, or whatever they're called, I always forget the name(((
The main units are Whispers with jetpacks and the Tau Commander. If you control the Whispers well, they can destroy buildings that have just started being built, thereby undermining the enemy's economy. The Commander is needed for support; it works well against Necrons, but it's mainly used to delay the enemy's advance on our base because the main fighting will take place there (this is only against Necrons) through slowing, turrets, and well-controlled Fire Warriors. At the decisive moment, we bring in the hero and destroy everything with the entire army. In general, I like Tau because they have more tactical options than other races. Okay, what do you think about countering Harass with different races?
Against Space Marines, the Assault Marine is decisive because of close combat and, most importantly, the jump ability. But if Harass is done with Whispers, Assault Marines will be ineffective.