They're not good. The natural adventure mode for new DMs is the lazy railroad. Sandbox adventures are not natural. "Play to find out what happens" is not natural.
That's not my experience. The first 20 or so published modules* I used (they weren't called adventures) presented locations, foes, NPCs, and maybe some environmental effects. The rest was up to me. It wasn't daunting or difficult. When PCs interact with monsters, NPCs, and dangerous environments, interesting things tend to happen. And for every published module I used, I made up one, using the same model of setting, maps, foes, and PCs and letting the story happen organically.
It wasn't until I bought the Rahasia module in 1984 that I saw the first scripted adventure. By then I had been DMing for five years and run dozens of published and homebrewed adventures. And Rahasia was like no other module I had ever seen. The story was already written! That seemed so




ed up. Even though I was still only in 15, and money was tight, I set aside Rahasia and never used it. When it became apparent that the scripted adventure was the new default model, I stopped buying published adventures altogether and made my own content. I didn't buy another published adventure for over a decade.
So if all you've ever seen is railroads, they may seen like the natural adventure mode. But they seemed like a unwelcome aberration to me the first time I came across one.
* Including In Search of the Unknown, White Plume Mountain, Hidden Shrine of Tamoachan, The Caverns of Thracia, Steading of the Hill Giant Chief, and other modules now regarded as classics.