With the update to the codex back in April, Tau have gone from a mediocre dated army that only a handful of diehard players use to a top tier army that can really pack a punch. The basic mechanic of the Tau hasn't changed a great deal. They still deal a lot of damage at extended range and their basic weaponry is S5 instead of the usual S4. They still suck in combat (for the most part) and the majority of the heavy firepower comes from crisis teams.
Unusually in 40K, Tau have a mixture of toughness and strength values. Their standard profile is T3/S3 but their suits are mostly T4/S5 (or T6/S6 for riptides). However, their other stats are pretty uniform with most units having BS3, WS2 and Initiative 2. The most notable exceptions to this are commanders who have improved BS/WS as you might expect.
Strengths and Weaknesses
On the face of it Tau are a gunline army. However, don't expect them to simply sit back and shoot. Of course they can do that, and do it well, but their real strength lies in their unexpected mobility. The main things that set Tau apart though are a prevalance of both Skyfire and Interceptor, Ignores Cover thanks to Markerlights, Storm of Fire from the Ethereals (making Fire Warriors devastating at close range) and Supporting Fire. The last one of that list has gone some way to addressing their weakness in combat.
There's no doubting one of the main weaknesses of the Tau is combat. However, you may not get there. Slogging your way on foot towards a Tau gunline is pretty much suicide but dropping in from the skies or outflanking won't help you much. Expect your reserves to be targetted as soon as they arrive. If you can get into combat then they'll fold pretty quickly but it's worth thinking about Supporting Fire. Manipulation of Overwatch is pretty key for defeating Tau, more on that below though.
There are two other important weaknesses too. Firstly, Tau troops really aren't good at taking objectives, never mind holding them. You can throw fire warriors in devilfish but they're pretty expensive which takes away valuable points for other toys. Kroot may be able to Outflank but they fold under the slightest pressure. The second weakness is their ability to deal with AV13/14. The majority of their firepower is either S5 or S7 and their melta weaponry is limited to select units.
5 Units to Watch Out For
The combination of interceptor and skyfire is also pretty deadly. Some Tau players will incorrectly try and fire both weapons in their opponents turn when actually a multi-tracker only allows for two weapons to be fired in the Shooting Phase. Even so, position your flyer within 9" (melta half-range) of a riptide and be prepared to lose it. A lot of Tau players are now taking 2 or even 3 riptides due to their ability to dominate flyers and deep strikers alike.
There are some horrible lists with broadsides in. In the previous codex Broadsides wielded S10 AP1 railguns but they've down been downgraded to S8 AP1 heavy rail rifles. However, they also have the option of the S7 high yield missile pods and these are what you're likely to find most broadsides equipped with. The choice is then between Skyfire and Interceptor (a broadside can't have both as standard). Most players go with Interceptor and rely on twin-linkage and shot volume to compensate for Snap Shots against flyers.
The missile pods are pretty short range but it isn't difficult to position them so that they can cover most of the board. Broadsides can also take missile drones for even more S7 fire at lower BS. For this reason they're often joined by a commander to boost the drones to BS5. The commander can also be given upgrades to allow a squad of broadsides to ignore cover and/or re-roll armour pens (or wounds against MCs). They can also fire their S5 secondar weapons at the same time. Each squad of suits can put out a sickening volume of fire that few opponents can withstand. All this from a suit with two wounds and a 2+ save.
3. Crisis Suits & Commanders
Whilst crisis suits are incredibly versatile with a wide array of different weapon options the most commonly used weapons are missile pods (more S7 fire), plasma rifles (S6 AP2) and fusion blasters (18" meltas). They often deep strike into enemy lines to enable them to bring down vehicles and heavy weapons teams alike.
What they're infamous for though is their "jump-shoot-jump" manoeuvre. This allows them to hide behind cover, pop out to shoot and then hide again afterward. They can even do this on the turn they deep strike now. This makes crisis teams incredibly slippery. The commander can be given a 2+ save too which enables him to act as a tank.
4. Fire Warriors
Pretty weak but nevertheless their cheap as hell. At just 6pts a piece most players will take these over fire warriors for the simple fact that their a scoring unit that doesn't prevent them from affording more riptides and broadsides. The key trick for Kroot is to Outflank onto objectives and hide out in cover. They can also deal with weaker units like cultists, guardsment, termagants who may be holding objectives. It's important to remember this when placing your objectives.
How to Beat Them
Tau can really hamper your gameplan, especially if that plan relies on flyers or deep strikers. However, the key is to adapt your playstyle to compensate. For starters it's worth bearing in mind that their Troop choices are weak and pretty easy to take down. As with any GEQ army flamers (particularly heavy ones) are your friend. It's easy to overlook the troops when you've got to concentrate on things like riptides but with most games based around objectives you should concentrate on their troops if you can.
Taking out those broadsides can be a pain. They can be dealt with pretty easily by things like TH/SS terminators but any decent Tau general will priortise them as soon as they appear. However, a land raider (which some Tau armies would struggle to take out) with some terminators inside, may be a solution.
Markerlights are a big deal for Tau. In a similar way to Imperial Guard orders, if you can take out the source you can drastically reduce the firepower. Pathfinders are pretty easy to deal with and, although slightly more slippery, marker drone teams aren't too difficult to deal with if you can get them at an angle where the commander can't tank.
With Tau there's a tipping point. Expect to lose a lot early on but in later turns as you get into combat you'll start to turn the tide. The key to getting into combat is manipulating their Overwatch. Trying to approach a Tau gunline head on is foolish as theres likely to be a network of Supporting Fire. However, by picking off units on the edges of the formation you can minimise the amount of fire coming your way. If you're charging with multiple units the order in which you declare the charges is important too.
Tau can be incredibly tough for some armies to beat. There are some top level builds that are pretty difficult for any army to deal with so don't be disheartened if you lose to them. The important things to remember are that their troops are weak and that you shouldn't feed your units to them one at a time.