Infants carried with their face to the front have adverse effects on their growth. Children often taken with a front-facing are more susceptible to their growth and development and make them anxious in adulthood, similarly if you use the stroller that faces the front.
Babies find it difficult to get their parents' attention and rarely speak with them. Babies are more likely to suffer from stress, even trauma. Conversely, children who're carried traditionally tend to laugh, listen to their mother and fall asleep while facing her. It shows lower stress levels.
For many babies, life in carriage is poor in conversations and makes stress. The babies will grow into depressed, anxious adults.
At a very young age, children brought facing forward will see the very busy activities of adults. Imagine you're tied into objects and can't turn away from a very crowded situation. Slings overlooking the front and prams create a very stressful situation for the baby.
However, as long as the baby is loved and fed, how to hold it won't affect its health. For newborn babies, it may affect their health, but this doesn't apply to infants older than 3 months.
I think there's nothing wrong with letting them see the world before the age of 12 months.
You may also like:
A Headbanger's Ball
Man with a Pram
The Rocket Powered Pram
From Pram to Primary School
A Contented House with Twins