It is true that portable chargers can be bad for your iPhone, in the sense that they may cause damage due to the power of a charger. However, I am not sure why this would happen but it has been said by some sources such as Apple Support Communities and MacRumours.com that charging an iPhone with certain brands of portable chargers (e.g., Anker) can lead to permanent battery damage on iPhones which cannot recover even after being fully charged again..