Robust grammars yielding a deep syntactic analysis for unrestricted text remain yet to be developed.
Purely statistical approaches to shallow parsing usually allow for relatively easy learning mechanisms and efficient parsing but they lack transparency and have inherent problems like dealing with long distance relations or recursively nested structures. Additionally, large manually annotated corpora are usually required for training - so they also require the application of explicit linguistic knowledge. The approach of manual grammar coding has to face the serious problem of managing the resulting rule-system. Manually coded grammars often reach a level of complexity that makes them hard to maintain, to extend or even to understand. Especially with grammars aiming at high coverage of realistic texts.