What problems might there be with using Viking myths as historical sources?

The problems of using Viking myths as a historical source are the same as using any myths as a source: they are not reliable: they were first transmitted orally and thus had many opportunities to be changed, inadvertently or not, and they could be biases, as only the winning sides usually live to tell the tale (if there was a war, only the winning side could tell stories about it, and an even later person in power could re-write the history, as it has happened many times in pre-Columbian America.).