it's a tradeoff between more keypresses vs. more finger/arm movement; also the topic is pretty contentious and nobody will give you an unbiased answer. My biased opinion is that layer-based boards without thumb keys are horrible, something like the Atreus is exclusively better than the Planck.

The Planck was designed to be small, not ergonomic. The popularity of the layout is (imho) due to good market timing, low cost, competent business operation, and good electronics design work.