He could cut them, but it was way harder, and you were still wide open to getting hit. But if you look at old CT match vids, you'll see Haku's j.2C'ing Nu's 214D as they came in and stuff sometimes.
I don't think anyone ever cut lobelias though.
The neat thing is it would nullify the entire attack though. The void doesn't stop the entire attack, just the parts that are in the bubble, so it's easy for Haku to still get hit in the parts that are outside of it. But when you cut a wave of swords from Nu in CT, it would just nullify the entire attack and you could just watch it go through you. The tradeoff is of course that the bubble sticks around to nullify other attacks.