If you think about how we all naturally sit, it is on our bums with our legs supported. It is a great position of babies/toddlers as they are properly supporting their spines/hips and developing core muscles. (I was just talking with a PT about this last week). If I am in a chair that does not support my legs, they fall asleep. Occupational health and safety standards have a strict time limit that workers can be in a harness (like a bjorn).
A bjorn was our first carrier - I found it uncomfortable on my neck and shoulders, even with my 8lb newborn. When I really thought about the carrying position, I realized it probably wasn't the comfiest for dd either.
Whether an ergo type carrier, or mei tai, or ringsling - the most comfy position for babe is on their bum, with their knees slightly higher than the bum and legs supported by the fabric from knee to knee. There are lots of great carriers (in lots of good price ranges) that will have this positioning, and be way more comfortable for you. As a bonus - the upper weight limit is usually 35-40lbs - so a much better investment than the bjorn (as mine sits sadly unused in the garage

).