위험스럽다 and 위험하다 both mean "dangerous". 위험하다 has a more direct feel and wider usage, but the difference is not much.
-스럽다 is a suffix which attaches to nouns indicating certain quality or trait. So -스럽다 is similar to "-rous" and "-ish" in English. But the tricky thing is that not every such noun takes this ending, and there is no simple rule as to which can or cannot take this form.
-하다 also functions like -스럽다, but it much more frequently forms the ending of action verbs. So you can''t blindly take it like -스럽다. Whether it means a quality or an action depends on the stem of the word.
Here are some examples:
- 사랑(n) = love. 사랑스럽다(a) = loverly, lovable. 사랑하다(v) = to love.
- 걱정(n) = worry, 걱정스럽다(a) = worrisome, 걱정하다(v) = to worry.
- 위험(n) = danger, 위험스럽다(a) = dangerous, 위험하다(a) = (same).
- 경망(X - not used separately) = frivolity, rashness, 경망스럽다(a) = frivolous, 경망하다(a) = (same).
- 잔인(X) = cruelty, ruthlessness, 잔인스럽다(a) = cruel, ruthless, 잔인하다(a) = (same).
- 안전(n) = safety, 안전스럽다(X), 안전하다(a) = safe. (some adjectives don't take the -스럽다 ending)
- 엄격(X) = strictness, sternness, 엄격스럽다(X), 엄격하다(a) = strict, stern.
- 곰살(X) = soft and caring manners. 곰살스럽다, 곰살맞다 (a), 곰살하다(X). (some words don't take the -하다 ending)