Ibn Majah was the last of the six Sunni collections to be treated as canonical and the one whose place has been argued about longest. The reason is a single feature. A large number of the reports in his Sunan appear in none of the other five, and specialists compiled lists of these additions, the zawa'id, precisely because they mattered: a report found only here is either a genuine survival that the other compilers missed or a weak one that they declined. Critics who examined the additions graded a considerable proportion of them low, and that assessment is the substance of the whole dispute.
The value and the objection are therefore the same fact. A collection that duplicates the others adds nothing; a collection that adds material is by definition adding what others left out, and the reasons they left it out have to be weighed one report at a time. He arranged the book for legal use, chapter by chapter through the topics a court has to decide, which is the standard form for a Sunan and shows what the compilation was for. He came from Qazvin in northern Iran and travelled the ordinary scholarly circuit through Iraq, the Hijaz, Syria and Egypt to gather what he gathered.
The compiler himself was not what the criticism was aimed at. He was recognised as a serious collector by his contemporaries, travelled and studied in the ordinary way, and wrote in addition a commentary on the Quran and a history of his own region, neither of which survives complete. The dispute concerns editorial policy rather than competence, and the discipline is generally careful to keep the two apart.