Most of the RME HDSP/HDSPe PCI/PCIe cards work on Linux using the ALSA snd-hdsp or snd-hdspm drivers. The ADI-2 can use the generic Linux USB audio driver AFAIK.
Many, many production-grade devices (another example: the Behringer (Midas) X32) work right of the box. The fact that the iPad did not allow drivers forced almost all audio interface manufacturers to clean up their act and get fully compliant with the USB audio specification. The side effect of this is that they Just Work (TM) with Linux too.
(*) the one issue is that this particular device requires a specific version of the firmware to avoid a bug that only shows up on Linux. This is not true in general.
You can still use Linux, but you won't be able to use those plugins.