The xr18 and x18 offer good value if someone needs that many channels or needs the capability to record 18 tracks. a few years ago, just an 8 channel firewire interface was as much as the x18 is today.
The price of the x18 is lower than the xr I suspect because most appear to be opting for the rack mount design of the xr18 instead of the desktop mixer design of the x18. They have basically the same functionality.
The PC edit software is very much like using a daw program like Reaper. The android software is different, but functional. So there isn't a steep learning curve.
It might seem like a jump to spend $249 for the xr12: instead of $99 for the Mackie, but you are getting more inputs, wireless control, compression, gates, reverbs, eq and RTA functions not to mention 2 track recording direct to USB. You can sit in your lawn chair and control the xr across the yard with your tablet.
Or make the further jump to the x18 if you want more tracks and multitrack recording to your PC or other computer.