The hand-made SD card cartridge reader shown in the original Game Boy Zero guide could be incredibly picky and finicky. If your wires weren't just the right length, or you happened to have an SD card that didn't like it, it could make your Pi refuse to boot make it read/write unreliably. These boards from prerunnerseth have built-in filtering so they work with just about any SD card you throw at them!
•Hard gold plated Fingers for long life over many removal in insertions of the cart
•Advanced Filtering for Data Reliability and Wiring imperfection Tolerance.