I think deforestation-based carbon credits are a particularly easy place for scams, or even accidental double-selling, or selling acres that wouldn't have been cut down to begin with.
I think buying credits from GoldStandard [1] is a safer bet. While I'm sure there's some of the same shadiness occasionally, I think that a lot more work goes into vetting projects. The majority of the projects I fund are small solar, wind or hydro plants. While many of these plants might get built anyway, I think the funding from GoldStandard may make the difference for lots of these small projects.
1. https://marketplace.goldstandard.org