I'm going to explain why the Tau Heavy Support slots are not worth taking.
We all know railguns are awesome. Broadsides are great, Hammerheads are still pretty good and more versatile, etc. Every army fears the S10 AP1 from basically infinite range, right? I want to make the argument that both broadsides and hammerheads are inferior to other options, and that an optimized Tau list does not need any heavy support units at all - in fact, it can spend those points far more efficiently elsewhere!
I began by looking at equal costs of broadsides to crisis suits. I used the standard ASS Broadsides and compared their performance against Deathrain Crisis suits (Twin-Linked Missile Pods and Targeting Array) and Sunforge (TL Fusion Blaster and Targeting Array) Crisis suits.
At equal costs, you get 2 TL railguns from the broadsides, compared with 6 TL missiles from deathrain suits, and 3 TL melta shots from the FB suits (I assumed they were in melta range, not a difficult thing to achieve with deep strike and general crisis suit mobility).
Against av10, for both ground targets and flyers, Deathrain suits beat out Broadsides in effectiveness. Some stats: 2 Broadsides average 1.5 pens, 1.5 hp in damage, and a 63% chance of killing, each turn they shoot, against av10. 3 Deathrain suits average 2.6 pens, 3.5 hp of damage, and a 38% chance of killing, each turn, shooting at av10. Because of glancing hits, the Deathrains are very high-percentage av10 killers compared to Broadsides.
Against every higher armor value, for both flyer and ground targets, Sunforges in melta range beat out every other option including broadsides. Some stats: against av13, 2 Broadsides average 0.75 pens and a 37% chance to kill each turn. 3 Sunforge suits in melta range average 2 pens, 2 hp, and a 75% chance of killing each turn. Against av14, Sunforges are better by a 2:1 margin compared to Broadsides of equal cost.
It's also worth noting that at av12 and lower, 3 Deathrain suits performed even or better than 2 Broadsides and cause an average of 1 hp per turn against av13 (same as broadsides - but the broadsides have 0.75 pens per turn as well, adding a 37% chance of destroying the vehicle). Broadsides are better than Deathrains against AV13, but still not as good as Sunforges in melta range, and Deathrains can still do alright.
I didn't test Hammerheads because Broadsides are considerably better against all armored targets (2 twin-linked railguns vs 1 non twin-linked railgun at the same points cost), and taking Hammerheads for an AP4 large blast isn't worth it alone, since all Tau infantry are superior at killing other infantry. (compare the Hammerhead to a Leman Russ, both have the same point cost, and you'll see how bad the hammerhead is, with the exception of its mobility and survivability with disruption pod and multitracker). Sky Rays perform worse in all aspects from the railgun or Suit options and should be avoided.
So in pure shooting power, Broadsides and hammerheads lose to crisis suits hands down in all situations vs armor. Fusion Blasters in melta range were more effective at higher armor values, while Missile Pods were roughly equivalent, better or only very slightly worse than Broadsides up to av13. Facing armor values of 13 and lower, Deathrain suits are pretty clearly superior to Broadsides and Hammerheads. And these two HS units exist to destroy armor! av14 is the main problem to concern ourselves with, and fusion blasters in melta range deal with that far better. But pure shooting power isn't the whole story, of course. So what are the other differences we need to take into account?
Railguns fire from much farther away, and are attached to more survivable units. You can't deny a 2+ save and 72'' range is going to keep broadsides alive longer. The tradeoff is clear: a broadside stays alive and keeps firing for longer than a Crisis suit (particularly the fusion blaster variety), but kills things slower. Is this a tradeoff you want? I would argue no. The longer enemy armor stays alive, the more chances it has to earn back its point cost kill your valuable units. Compare killing a riflemen dreadnought or a land raider after 1 or 2 turns, compared to after 3 or 4. Over those extra 2 turns, that vehicle is shooting, transporting, or doing other useful stuff for the enemy. Every turn it's earning back points. Anything you kill before it's had a chance to do anything is points wasted for the opponent. I don't believe the range and survivability trade off is worth the lowered firepower and point-efficiency.
An advantage Crisis suits also have is maneuverability thanks to jump-shoot-jump. Only Hammerheads are as maneuverable with a multitracker, but with firepower that isn't really close to Crisis suits.
Finally, we need to address an important point: fusion blasters require melta range to be so much more effective against av14. My response is to say that deep-striking, especially using a marker beacon to re-roll scatter, is pretty reliable. av14 vehicles are expensive and won't be very numerous in most lists you'll come across, with the exception of IG (a more balanced Tau list might include more FB teams to deal with this). Typically, deep striking a single fusion blaster team and taking out an av14 vehicle in a single turn will earn back their points and then some, and you can then let them suicide or annoy the opponent without much worry afterward. Particularly favorable circumstances will see your Crisis suits able to use a jetpack move in the assault phase to retreat behind cover or out of los, directly after deep striking.
By maxing out Crisis suits and forgoing all heavy support, you're using your points more efficiently. After some math hammer work, I believe Tau heavy support choices are all inferior uses of your points, compared to crisis teams.
So what kind of army am I advocating for, if we aren't including Hammerheads, Broadsides, or any other heavy support choice? Essentially, lots of Crisis suits, and lots of troops, with Pathfinder support. Both Fire Warriors and Kroot are cost-effective at killing any enemy infantry in rapid-fire shooting ranges, and each have their own unique bonuses and are slightly better at killing different things. Focusing on a lot of troops makes holding objectives much easier, too, and objectives really are the name of the game.
Thanks for reading, and stay tuned for specific army lists and playtests as I examine this theory further!