With SailfishOS 5, we got the rotating homescreen, so you can enjoy your apps in landscape mode. Some apps lack this feature, this thread lists them. Maybe we can find solutions to fix it ourselves (by patches, forks…etc), or the author can add this feature when they see there is demand for it. That way, we could have a seamless user experience.
Deadbeef Silica Music Player Landscape mode only hardcoded in equalizer-setting-page, others portrait
The sudoku game Aenigma is a bit curious - landscape is supported in some pages but not others.
The first page (which simply says"Pull down to start a new game") works in landscape, as does the game screen itself, but between them is a screen where you choose the difficulty and other options - and that page is portrait-only. The stastistics page does not work in landscape, but the about page does.