You can assault Tau, but it's all about timing.
Night shields can be handy against them, though most of their units can move 6" and shout their weapons meaning if whatever you shot is still alive it should be able to shoot you back. Riptides, hammer heads an skyrays are also unaffected by night shields (and are popular choices). Tau can also be highly mobile depending on how you build them invalidating the night shield. The effectiveness of night shields really depends on the tau list.
Here are a bunch of reports (with pictures) against the new Tau, that might be helpful. They cover a variety of Tau armies and styles.
BR21: The Black Buzzards VS New TauBR22: The Black Buzzards VS Tau (Rematch)BR23: The Black Buzzards VS Tau Mech/RiptideBR25: The Black Buzzards VS Tau Mass BroadsidesBR28: Kabal of the Bitter Envy VS Tau MobileHope that helps.
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Latest Report: BR4: The Repugnant Ramblers Vs Imperial Knights - 1250pts
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“Even the Black Buzzards thought highly of him, and those maniacs were renowned for hating everyone.” - Tantalus, by Braden Campbell