I believe it is often cited that dependency grammars are more adequate for free word order languages. How actually, is dependency grammar more descriptive or less limited, than constituency grammar, in describing free word order language texts? can you provide few common cases where constituency grammar would fail to capture the grammar of a sentence in a free word order language?!
Dependency structures tend to be flatter than phrase structures, which produces fewer opportunities for discontinuities to occur in the syntax. These flatter structures are the reason why many dependency grammar (DG) people claim that dependency is better than phrase structure (i.e. constituency), at addressing free word order. However, dependency is actually no better or worse than phrase structure at addressing the syntax of languages with relative free word order. The two approaches are actually equivalent in their potential to address word order. This equivalence is due to the ability to translate any dependency structure to the corresponding phrase structure -- Note though, that there are phrase structures that cannot be translated to corresponding dependency structures, which means dependency and phrase structure are NOT notational variants of the same thing.
Typical phrase structures are more layered, i.e. taller, than the corresponding dependency structures. Consider an example in this regard, a simple German sentence with a transitive verb (N = nominal, S = sentence, V = verb, VP = verb phrase):
The phrase structure analysis assumes a VP constituent, which is not unusual for many phrase structure approaches to German sentence structure. The presence of the VP constituent means that the phrase structure shown has three layers. In contrast, dependency does not and really cannot acknowledge a finite VP constituent, so the dependency analysis has just two layers. The dependency structure is hence flatter than the phrase structure.
The advantage that the flatter analysis has becomes apparent when one alters the word order:
The hierarchy of words in the dependency analysis remains consistent despite the change in word order. There is one change only: the two nominals N1 and N2 have simply swapped positions. In contrast, what the corresponding phrase structure analysis for the sentence should be is not immediately evident. To achieve the word order shown, the phrase structure account would need at least two changes: the VP constituent would have to swap positions with N1, and then within the VP and the V and the nominal N2 would have two switch positions.
The difficulty facing the phrase structure account in this case is due to the presence of three layers instead of just two. Phrase structure syntax, however, does not in itself necessitate such layered analyses. If one converts the dependency analysis in (1a) directly to phrase structure, one gets the following tree:
This tree is a manifestation of phrase structure because the number of nodes (four) outnumbers the number of words (three). But there are only two levels now, which means the ability to address the alternative word order is the same as for the dependency approach:
Just one change is necessary, just like for the dependency analysis in (2a). The problem for many phrase structure grammars in this area is that they build on the presence of a VP constituent. If in fact no such constituent exists, then a crucial pillar of support for phrase structure syntax in general disappears and there is less reason to choose phrase structure over dependency.
In sum, the reason why DG people claim that DG is better for addressing the syntax of free word order has to do with the flatter dependency structures. Flatter structures provide less opportunity for difficulties to arise when confronted with alternative word orders. The issue of flat vs. layered structures is addressed in more detail in Wikipedia: