Mixed mode surveys have shown to attract different types of respondents. This may imply that they are succesful. Internet surveys attract the young and telephone surveys the old, so any combination of the two can lead to better population estimates for the variable you’re interested in. In other words, mixed-mode surveys can potentially ameliorate the problem that neither telephone, nor Internet surveys are able to cover the entire population.
The bad news is that mode-effects (see posts below) coincide with selection effect in mixed-mode surveys.