In fairness, one of the things that happens is that expert witnesses get chosen based in part on past record, writings, etc. When I wrote a report, I wasn't pressured to write anything I didn't believe. However, I had previously written a public research note that laid out my take on the case pretty unambiguously.
I don't necessarily think it's as direct as "cross lawyer X, never work for lawyer X". I think past expert testimony is a big part of that past record that lawyer Y will look at, and people who want to become professional expert witnesses know that.
