We can't know for certain. That's the idea of systematic biases. There no way to tell if all your trials are slanted in a specific fashion, if the biases also appears in your high quality studies.

On the other hand we have fields such as homeopathy or telephathy (Ganzfeld experiments) where there are meta-analysis that treat all studies mostly equally that find that homeopathy works and telepahty exist. On the other hand you have meta-analysis who try to filter out low quality studies who come to the conclusion that homeopathy doesn't work and telepathy doesn't exist.