I think it depends on what your goal is. In my case, I removed almost all the visible rust with rust and paint stripping disks on a 4.5" grinder. So I only needed the converter to deal will little bits in pits or sheet metal folds, like at fender reinforcements and door skins. For that I used Ospho. I've used Picklex 20 which is really good, but come at a premium cost!
That was followed with a quick hit with 80 grit and epoxy primer. In the door skin and fender lap areas the primer was literally poured in as I had the parts on a table so I could roll it around to get it into every nook and cranny.
I don't trust any rust converter to work if the rust hasn't been removed first to a great degree. Sanding, grinding, stripper disk, acid or something needs to get to the metal. Because, the primer bonds to the converter, the converter supposedly bonds the converted outer layer of rust to the substrate. And in all the tests I've done and seen online there is always red rust between the metal and the black converted layer. If the rust de-bonds from the metal, the 'sandwich' will eventually fail, IMHO.
Just don't trust that.