Curtain rods - round, fixed only at the ends, unsupported in the middle. They're a trampoline in the middle of the range which, unfortunately, is where we work most of the time.
Next step up is the supported round rails which are better but still not great.
Next step up is square linear rails which are supported along the whole length and whose bearings are set up the best of all three for making sure the ONLY movement possible is along the rail (not twisting in any direction or allowing displacement sideways or up and down).
Now, throwing four of them in there might make up for it. But I wouldn't be going there if I wanted a machine that would do more than light cuts in balsa.
Don't forget that, additionally to that thousand bucks, you need to get servos, drivers, breakout, an upgraded spindle, VFD, hardware to hang it all together, cables and home stops and connectors, a housing to put the electronics in and so on and so forth. By the time you add all that up and factor in your frustration getting it going the X6-2200L I got starts looking like good value again.