Problems with the stock carriage:
The standard X carriage on my FLSUN i3 did not travel evenly on the extrusion. On certain points it would raise/lower the entire carriage ever so slightly.
The results are uneven lines along the X axis, especially on large rectangular parts. During operation this behavior is only visible under very close observation. An easy way to determine wether your X carriage suffers from this issue is auto-bed-leveling. The probing results will show a wave pattern along the X axis while the values along the Y axis roughly stay the same on the mesh.
0 1 2 3
0 -0.1 +0.2 -0.2 +0.3
1 -0.1 +0.2 -0.2 +0.3
2 -0.1 +0.2 -0.2 +0.3
3 -0.1 +0.2 -0.2 +0.3
*These are fictional values to visualize the problem.