+1. Charging packs in series unbalanced is particularly risky, as any difference in charge state (highly likely) can easily lead to overcharged packs. Charging in series with balancing can significantly increase charge time to accommodate the balancing, and requires very careful hook up.

Parallel is not without limitations either, ideally packs should be reasonably close in charge state, but seems to be the way to go. Either requires enough charger power and capability to make it worthwhile.