One thing I'd want to know about a self-built emulator is who checks it. If the same model that writes the firmware also writes the peripheral models from the datasheet, a misreading of, say, an SPI FIFO flush or a timer prescaler gets baked into both sides, and the tests pass for the wrong reason. The errata-level quirks the founders mention are exactly what a datasheet-derived model won't contain. The practical test is differential: capture a few traces from a real board (logic analyzer on the bus), replay the same stimulus in the emulator, and diff the cycle timing and register state. Without that, I'd treat green results from a homemade sim as unverified.