>>96876361
The basic Tau strategy is essentially:
>Use long-ranged railgun artillery/carrier ships to snipe at incoming Imperium ships
>Use Kroot warspheres as their close-up ships to keep the enemy tied up in a brawl (much like how the Kroot themselves are used)
>Try to kite the enemy as much as they can
This can work pretty well, and they often take out more Imperium ships than they lose (and Tau ships are easier to replace in the long-run than Imperium ships are in any case).

However they run into a couple problems. Namely, from Space Marines, who have some very fast ships and are able board Tau ships, which poses a problem since the Tau aren't the best in CQC. Sure, they might have some Breacher squads and possibly Void Battlesuits depending on whether you take Battlefleet Gothic to be canon, but they're still not gonna be as good as Space Marines in a CQC brawl.

The other problem is that the Imperium has had 10,000 years of relative peace to build up a massive space navy, so even if the Tau's manufacturing capabilities are superior on a per-planet level, it can't really compare to what the Imperium has stored up over the millenia (before someone comes at me I said 'relative' peace, the Imperium never had to face the challenges during the Great Crusade that the Tau are facing during their expansion).

>>96876369
>You're talking about a faction with a million planets
Who is heavily reliant on a million Space Marines to get stuff done.

That's the problem the Imperium has. They're big sure, but not where it counts. They need a trillion space marines, they have 1 million or so. Hell, the Imperium has hundreds of billions of Guardsmen fighting the Tau right now and making no progress. I don't think 'throw more guardsmen at the problem' is going to help them.