Your comments seem entirely reasonable. Would you not expect that the
degree to which this is a problem would depend on the degree of genetic
redundancy; that is, in groups of organisms that typically have multiple
genes for necessary functions, particularly 'bad' mutation barriers may
be relatively easy to cross, whereas those eukaryotes (of which there
are some) and bacteria (most) that tend to have single copies of most
genes would very rarely be expected to cross such barriers.
