In my games, even though I'm a relative newcomer, the Guardian Angel beats almost without losses on week 3 in 90% of cases, and on week 2 in 20% of cases.
In general, there's some wild off-topic discussion going on.
The problem with mages without restrictions isn't that they'll be chosen as starting heroes, but that if you get a mage as one of your heroes, it's 99% a rush on week 2 - hit-and-run with the mage, and then finishing off with the main hero. And that's what makes it overpowered.