Whether a baby carrier is comfortable or unbearable comes down to load path. The weight has to reach the wearer's skeleton somewhere, and design determines where.
Shoulders or hips is the central design question
A carrier with narrow straps and no waistband transfers the entire load through the shoulders and upper spine, which is a poor structure for carrying weight over time.
A structured waistband transfers much of the load to the pelvis instead, which is designed to bear weight. The same child then feels considerably lighter.
This is why comfort differences between carriers are often dramatic at identical weights. The load did not change; the route through the body did.
Distance from the body multiplies the load
Weight held away from the torso creates a turning force that back muscles must counteract. The further out, the harder those muscles work.
A carrier that allows the child to hang loosely therefore fatigues the wearer far faster than one holding them snugly against the chest.
This is the reasoning behind the common instruction to tighten until the child is held close. It is a mechanical adjustment rather than a comfort preference.
Hip position is a separate consideration
Positions in which the legs hang straight down with the whole weight through the crotch concentrate load at the hip joints in a way developmental guidance discourages.
Supporting the thighs to the knee spreads the load across the legs and holds the hips in a spread, bent position, which is the position paediatric bodies generally describe as preferable.
Hip development in infancy is monitored clinically, and any concern about a baby's hips is a question for a doctor rather than a matter of carrier selection.
Airway position is the acute concern
A young baby lacks the neck control to reposition their own head. A chin pressed to the chest narrows the airway, and the baby cannot correct it.
This is why guidance emphasises the face being visible and the chin off the chest, and why slings that curl a baby into a deep C shape are treated cautiously.
These points apply most strongly to newborns and to any baby who is unwell, premature or has low tone. Specific advice for those situations belongs with a health professional.
Fit changes as the child grows
A carrier set correctly for a newborn is wrong for a nine-month-old. Panel height, seat width and strap length all need revisiting as the child changes shape.
Many carriers are adjustable across a range and are used at one setting throughout, which is where much reported discomfort originates.
Back carrying introduces different considerations again, including when a child has the trunk control for it, and the manufacturer's stated limits are the relevant reference.