As I just mentioned, the concavity of a flat pedal (see above pic) is important to its feel and performance. To a degree this can be personal preference. Some riders like chunky pedals with a deep concave, others are okay with super thin pedals with virtually no concave to them (see pic below).
If you’re looking to get your first proper/decent set of flat pedals then I’d advise going somewhere between these two extremes ie. a pedal that’s less the 20mm thick and has a subtle bit of concave centre section.
Know more:- https://www.merlincycles.com/blog/buyers-guide-to-mtb-flat-pedals/
