That they can peg them to a currency like the USD. Unless you are the United States of America, that is literally impossible. But even if you discard that technical impossibility, none have even close to the assets required to even approximate a peg, so it is a scam both theoretically and practically.
Uhm, this is not very good advise, as the bridges especially often do not work well with non-Synapse homeserver implementations. And especially Conduit also has a very different way to setup appservices, so it becomes much harder to configure the bridges correctly.