Daʿf al-taʾlif is a weakness of construction where the arrangement runs against an established, widely recognized grammatical rule. The source describes it as خلاف القانون النحوي المشهور بين الجمهور—contrary to the grammatical rule well known among the majority.
Not every uncommon turn of phrase may be dismissed outright: linguistic evidence and legitimate grammatical disagreement matter. But a substantiated violation of accepted construction is the specific concern here.
This is mainly about syntactic relations, not heavy sounds in a single word or unfamiliar vocabulary.