ANSWER:
He was a great Imam and reformer and who cares what others said about him when his books and teachings are till date being studied in universities and his dawah has cleansed the whole Arab world from shirk and innovation with the grace of Allah?
https://islamqa.info/en/answers/36616/muhammad-ibn-abd-al-wahhaab-a-reformer-concerning-whom-many-malicious-lies-have-been-told