[throwaway7767]

I'm certainly not excited about the idea of all the random USB devices I plug into my machine having a PCIe lane with DMA capabilities available to it. Random vendor-provided USB mass storage sticks being able to read all the system memory? What could possibly go wrong?

In theory, IOMMU can mitigate these risks. In practice, barely any OS actually enables those protections, and AFAIK the CPU manufacturers (at least Intel) are still using availability of IOMMU as a differentiating factor for high-end CPUs.