How many times have "game used" jersey patch and bat relic cards been proven to be highly suspect, or downright fake? Several times.
Recently, even Eli Manning was caught selling fake used jerseys and helmets. Link here:
https://www.cnbc.com/2018/05/14/gian...d-helmets.html
If they're not 100% honest with GU jerseys, helmets, relics, and patches, I wouldn't trust autographs either.
There's two types of autographs: old ones, and the modern ones. The modern ones are easy. You can probably trust companies like UD to certify that they witnessed the athlete signing the card or piece of memorabilia. I don't think we have any reason to believe a serious fraudulent operation is taking place at UD or Steiner.
The old autographs are a different story. People are human, and just like TPA's they make mistakes. Because the athletes that signed the cuts are dead, there can be no witness to the signing, and someone at the trading card companies has to go out and bid/buy the autographs in the secondary market. This means forgeries will inevitably slip by due to the fact that the buyers at these companies don't have any magical authentication powers, and they can be just as fallible as the rest of us.
Do I think they're
purposely creating or buying fake autographs, or have in the past, to create insert cards? I can't make that assertion.