> Ok, but at least expert witnesses are constrained by the basic state of science in the field: They can certainly have a biased opinion but they can't go against established knowledge

How can you tell if you're not also an expert?

> the other party can also interrogate them and try to show holes in their argumentation

Yes, and when the science is beyond the experience of the jury, experts giving opposite opinions will be as hard to distinguish as conflicting non-expert witness testimony (or even the testimony of the defendant compared to the accuser or litigant).