Why can it be dangerous to overtake even on clearly visible, straight roads?

Your selections are correct. Even on straight, clear roads, overtaking can be dangerous because you might misjudge the speed of oncoming vehicles—they could be approaching faster than you expect. This makes it harder to overtake safely within the available distance. Also, the space needed to pass and return to your lane is often longer than it appears, increasing the risk of a collision. Both factors mean you must always carefully assess the situation before overtaking to avoid danger.