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I still haven't played it, but there are least a couple of obvious counters, imo...

Heavy Artillery Platforms, preferably with Prescience.
Grimoire Shenanigans (although this one is kind of a no-go if they have Runes of Warding, but that's a separate argument, RoW is the most broken ability in the entire game). Although even with RoW, 20 Demon Fast Attack of your choice with a 3++ are probably still going to put the hurt on it (and the Herald can challenge, smash one of the HQs, or get denied and use precision hits to kill hit and run)
Drop Pod Sternguard/Wolf Guard combi spam list (yes, the Tau will intercept some of them, got it)