I should actually have specified that Cyclones & Tanks don’t work well together “in a single engagement/battle” as I will elaborate on lower down this response.
Sorry, this post is going to get fairly long.
Bio-Tank is bad in a lot of situations.
Specifically, Bio-Tank is bad when the opponent has units that can regularly get onto the Bio (forcing friendly-fire, which is particularly deadly to the bio units), and it hemorrhages Tanks whenever the opponent has an army that can force Bio to kite or retreat long distances.
In TvZ, that basically puts Bio-Tank on a timer until Hive tech:
- Brood Lords turn Tanks into a straight up liability for Bio.
- Vipers can abduct and disable Tanks, rendering them useless. In order to keep Siege Tanks “useful” after that point, the Terran player needs a lot of long ranged units (Vikings, Thors, Ghosts) to ward off or punish/kill the Vipers, and a screen of either Hellbats or Widow Mines to keep the Tanks alive long enough to shoot if they get hit by Blinding Cloud.
- Fully upgraded Ultralisks are faster than Bio, tough to take down, and relatively efficient at killing Bio units with their splash damage. There may be a critical mass where Tanks just kill most Ultralisks before they can reach a Bio force, but otherwise this can easily fall into the friendly-fire or abandon the Tanks scenario.
In TvP the main friendly-fire threat is Chargelots, but Protoss will often need a lot of chargelots and maybe splash support to make that work.
In TvT, the Siege Tank is decent against all ground units, and it can be particularly difficult to make friendly-fire a significant issue for the opponent.
Pure mech/sky (a Hellbat line buffering for Tanks, Thors, Vikings, etc) doesn’t have quite the same problems that plague Bio-Tank. Hellbats have the durability to absorb a lot of damage (with no +armor bonus), and they have a fairly powerful burst to kill anything that tries to get on top of them or the units they are defending. Both attributes make Hellbats significantly less susceptible to friendly-fire than Bio units. The extra durability also buys Siege Tanks and other heavy mech/sky units more time to shoot and dish out damage; whereas Bio would normally be forced to retreat or take heavy losses. Mech is basically stronger than Bio in direct engagements.
To loop this back to Cyclones:
Cyclones are designed around kiting, and often have the same need/urge to retreat as Bio. They also need time to refresh their lock-on, and if they fight with their auto-attack they are likely to be near enough to the enemy that some enemy ranged units can directly target them over Hellions/Hellbats, bringing down the Terran player’s firepower at a faster rate. The Terran player would have better performance in a direct battle if most of their army could kite or if they had more Siege Tanks, Thors, Vikings, Liberators, etc which don’t share the Cyclone’s downtime.
Keep in mind that Battle-mech and mech can easily swap and reuse the Hellions/Hellbats, the vehicle upgrades, and the production between them. It isn’t particularly difficult to swap between one or the other, or to use them as separate forces.
Cyclones and Tanks don’t fit particularly well together in a single battle/engagement, but there is nothing stopping a player from using Cyclones for harassment, Tanks (along with Vikings, Liberators, Thors, and/or Battlecruisers) for defense & main-army, and moving the Hellions/Hellbats around as needed. That is actually how I usually play, with a harassment force of about 3-6 Cyclones along with some of the Hellions.