If the alternative to adding an additional interface is to just give DMA access to the device, I'm not sure I see the downside to using the additional interface

If the alternative to adding an additional interface is to just give DMA access to the device, I'm not sure I see the downside to using the additional interface. Even if the interface ends up being completely broken, at the very least there was something to break before you get DMA / RCE access. What possible interface breakage could trump free and unrestricted DMA access?