I'm pretty sure it's just your direct vicinity. Like objects you are holding or directly in contact with. . .
i.e. You cast heat liquid on soaking wet joe, it won't work, but if you cast it on a puddle joe is standing in, it just won't work directly in contact with joe, but it would heat most of the puddle, which will then heat the water next to joe's legs, etc, etc. . .at best it just offers you a chance to move before becoming soup.
I never got the feeling that those spells are intended to never be offensive, I just think they didn't want you to be boiling people's blood, or waiting for a rainy day to boil people in their clothes.
Like the teleported large rock squish. . . .
If you teleport a rock over my head, I'd say that odds are I'll have time to step away and avoid being squashed. . .unless I am unaware of you, or chained to the floor, or stuck climbing a narrow stairwell. . .
Creative use should have benefits, and sometimes that creativity should mean an almost definite kill. . . .if you set someone up to catch them perfectly, U spells are REALLY dangerous.