It is worse than than because of:

(i) negators aren't always next to the word they negate, to get good accuracy you need more of a parse (ii) sentiment is highly dependent on the domain. For instance, if one was looking at people's opinions on stocks, "Buy" and "Sell" are considered sentiment but these are emotionally neutral words in general. (iii) Also there is sarcasm, which sometimes people can't figure out right.