They obviously had reason to believe he was strong and dangerous, but there isn't good evidence that they had an accurate idea of his strength relative to themselves.

In my opinion their wariness comes from the fact that they have reason to believe he is quite strong from the bodycount of their peers that have encountered him. They just know that he is strong and that he is clearly dangerous. Hence they can't risk his strength.

I wonder how much they feared his Ta'veren effect. In my opinion he was more lucky than good in the first few books. But I don't recall any forsaken expressing wariness about facing him due to his being Ta'veren. I wonder why.