Down on the Cateran? I think you have a lot to learn about Ariadna. A Cateran is FAR superior to any line infantry with sniper rifles. No one fears unlinked LI ARO pieces. They'll just get picked off one by one. Cateran has marker state and a T2 sniper to protect him and threaten anything your opponent throws at you. If you want suppression fire infiltrators, Hardcases do that quite well for bargain basement points. Irregular units are valuable because they are so inexpensive and because you don't have to worry if they die.
Anyhoo, Tohaa have some excellent units, including Sakiel and Makaul. I don't think the Symbiomates are good for the game. But they are certainly not unbalanced enough that other factions, especially Ariadna, have a poor chance of winning. It all comes down to experience and skill. Lots of things feel overpowered the first time you lose to them, but over time you'll learn how to beat them.