Agreed. I saw it also happen for "reputable" stock brokers. They claimed no fees but always had a appalling exchange rate.
This applies to all goods and services. If everyone else is charging a fixed fee on top of a proportional one, and then one vendor advertises having no fixed fee, you can be certain that they're recouping it some other way - be it a steeper slope, non-linear pricing, or just a bunch of extra fees that will materialize after they lock you into a contract.