So what is Pythium's weakness?

[Pocus]

One of their few weaknesses is that they are not that versatile, when it comes to magic. The sole mage which has a random slot is the A-Theurg, costing a whopping 380 gold. Compares this to Arco. So you know what is coming at you (pretender excluded) when you deal with Pythium : air/astral mages.
Not that it does you much good, because air & astral are both very potent magics. But at least you can think lightning ward and antimagic as soon as you see them.