With sectional track, it is easier to get smooth curves in the smaller radii. But the big limitations of sectional track are limited geometry - you are limited to the curves available - and the multitude of track joints.
The Achilles heel of most track systems is the joint between sections. Rail joiners do not do a great job of holding mechanical alignment or electrical connectivity over time. Flex track minimizes the number of joints, and gives you freedom to to achieve any curvature you want.
To use Atlas flex track in particular, you need to learn to solder the rail joints on curves. Otherwise, the natural springiness of the track creates horizontal kinks in the curve at the joints. MicroEngineering flex track, OTOH, has little to no spring and holds a curve once it is bent, but you really need a form or template to pre-bend it to to get a smooth curve. In either case, you will also need a set of rail nippers to trim the rail ends of flex track.
Hope this helps