Before revision:  However, such problem setup has two inherent limitations that (1) it can only extract a flat list of information and (2) it assumes that the input data is serialized, often by a simple rule-based script. Nevertheless, real-world documents often contain hierarchical informationin the form of two-dimensional language data in which the serialization can be highly non-trivial .
After revision:  However, such problem setup has two inherent limitations that (1) it can only extract a flat list of information and (2) it is not suitable for highly structured information, which are nevertheless frequently observed in real-world documents often contain hierarchical informationin the form of two-dimensional language data in which the serialization can be highly non-trivial .