Since it explicitly models the grammaticality of the output via target-side syntax, the string-to-tree model (Xiao et al., 2010) significantly outperforms both the state-of-the-art phrase-based system Moses (Koehn et al., 2007) and the formal syntax-based system Hiero (Chiang, 2007).
there is a major limitation in the string-to-tree model: it does not utilize any useful source-side syntactic information, and thus to some extent lacks the ability to distinguish good translation rules from bad ones.