The central obstacles are weak composition, disharmony between words and obscurity (taʿqid). The component words must also satisfy the word-level tests.
DefectDiagnostic sign
Weak compositionArrangement violating accepted syntax
Word disharmonyCumbersome articulation arising between words
TaʿqidUndue difficulty recovering the intended sense
These categories are distinct. An expression can be grammatical but hard to pronounce, or both grammatical and pronounceable yet difficult to interpret.