The Decius is a recon unit with a light cannon. It was used as a countermeasure recon unit during the previous boss event.
Offense: This unit is great against any infantry unit except maybe Sand Vipers or Comrade Zero. It has just enough range to be good against Rocket squads and has a high enough rate of fire to kill them before they can become a threat. It also does reasonably well against helicopters but it should have support for taking them out and shouldn't be your primary unit for that task. They are very cheap with a quick recharge and are much more effective than other cheap scouts but they are definitely a support unit and wont survive at the front.
Defense: They can easily be killed by any tank or heavier recon vehicle. Longer range helicopters can also take them out fairly easily. They are deadly to your infantry so be sure to counter them with vehicles.
This is the long range variant and has the highest attack of the three variants.