The balance is problematic in every cycling directions app. Some may be a bit more comprehensive than others in certain areas but all of them prioritize shorter routes over flatter ones.
I wish there were a true flattest route app/website that prioritized quieter roads and more gentle slopes by default using existing terrain data instead of making users manually add a bunch of waypoints using their own knowledge in order to compare elevation profiles.
I know that OsmAnd has fairly granular routing preferences, including a "preferred terrain" setting with "hilly", "less hilly", and "flat" options (as well as a "use elevation data" option which I don't quite understand).
Magic Earth (proprietary app that uses OSM maps) allows you to choose the maximum hilliness you want your cycle route. I've never used cycle routing, so can't comment on how well it works.
I wish there were a true flattest route app/website that prioritized quieter roads and more gentle slopes by default using existing terrain data instead of making users manually add a bunch of waypoints using their own knowledge in order to compare elevation profiles.