Fr. Roberts was independent and had been for a while. In fact, how many here are aware that Fr. Roberts was posting on ex-Trad forums such Angelqueen and Ignis Ardens for many years? If I recall correctly he was even confronted online by the moderators on AQ and they accepted his explanation and gave him the benefit of the doubt. He has fooled a lot of people because he was allowed to rehabilitate himself and built up something of what you might call, social proof.
The SSPX would also be at fault for negligence as they never made Bishop Williamson aware of the situation. Anyway, Fr. Abraham was pulled from the Resistance circuit in Ireland once the information about him was known. Keep in mind, the state looked into his case and could not bring charges against him. That's a problem the summum ius types are going to run into, what if the civil authorities don't find anything or get it wrong?
Fr. Roberts was sent packing from Christ the King seminary for propositioning a seminarian. He applied to the sspx. Christ the King informed the SSPX of the issue. The SSPX accepted Roberts as a seminarian and later ordained him. All this while b. Williamson was in charge.
A LOT of people made b. Williamson aware of Fr. Abraham.
In past history religious hierarchy did not wait for civil authorities. They are took immediate action and such priests were sent to monasteries, never to be heard from again.