I tried both this summer and Mapy.cz is definitely better for hiking. You don't have to zoom in as much to see the trails, hiking trails are marked more clearly with major trails marked in a nice thick red line and it has relief maps (though they can't be downloaded) and the peaks are more clearly labeled along with their height. Also when you zoom out you see a height map instead of just grey for land and blue for water.

Overall it has a nicer attention to detail, for example dashed lines stay in one place as you zoom in/out instead of swimming around and I like the selection of downloaded areas better on Mapy.cz (it has flags next to country names and downloading a new area takes one tap instead of two or three) though it still feels flimsy.

I wonder why this app isn't more popular. Basically, there's a map app which is much better than Google Maps for a lot of use cases and very few people outside of Czechia use it. Is this a matter of marketing or is the app actually not that good?

I've been using it for a while and love it but intend to give the open source Organic maps a try. In particular sharing a way point in Mapy.cz seem to require an account with a Czech web portal.

The reason Mapy.cz has all the trails is because they also use open street maps.