Lie-flat doesn't mean the steepest angle
A newborn's seat should sit as close to a 170° lie-flat position as possible, so breathing stays easy and the spine takes load evenly. Too steep a semi-recline lets the head slump forward, which actually increases breathing resistance.
Don't overlook head and neck support
Choose a seat with an adjustable headrest and side wings to reduce movement from bumps in the road. A thin breathable liner can fine-tune the fit, but don't build it up so thick that the harness no longer lies flat.
Airflow and shade
For long spells outdoors, watch the shade and ventilation so the seat doesn't overheat. You should also be able to see a sleeping baby's face at any time — a carrycot or canopy with a clear view makes that easier.