Some open world games go overboard with these side activities and stray too far from the main storyline. Bethesda is aware of this pitfall and is actively engaged in preventing the feeling of being overwhelmed by the Radiant Story missions.
What pitfall? I like to stray far from the storyline
I guess they mean that they want to avoid you getting bothered with random missions ALL THE FRIKKIN TIME.
They probably won't tell you which quest is random and which is scripted because that will ruin the immersion, but random quests are bound to get repetitive at one point.
If there is a good balance between random quests scenario's and scripted quests, the random quests won't feel random at all until after you beat the game.
Random quests in Daggerfall were: go slay a harpy, go slay a werewolf, go slay an other werewolf... I see you're getting quite good at slaying werewolfs, so here's an other one. Congratulations, you got promoted. Go slay a lich and a vampire. I'm too old and too spoiled to fall for that again.