Thank you didu for bringing up such an important topic. Although I agree that its incorrect to write off a shidduch simply because the parents are bts I do think it brings up more for consideration: Two points which are critical which were already mentioned are having non-frum relatives and the difficulties it brings as well as the parents orientation to shidduchim in the frum society.
But I also believe its possible that althought the bt parents raised their children to be frum there still can be differences in a childs upbringing between the bt parents and the ffb parents. Certain hashkafos and ideas can really only be transmitted from birth and are very hard to explain to bts to become accustomed with. I’d esspecially be concerned if it was the parents oldest child.