For the most part, any slice produced by the Slicing Tool will be a perfect
replica of the region it
represents from the source terrain.
However, there are a two exceptions to this, inherent in the way Unity Terrains are implemented.
The first is with detail objects, such as grasses and plants. Unlike trees, Unity does not store exact data about where and how each plant or grass is placed on the terrain. Instead, it stores the density of each detail object in square sections on the terrain, the size of which is determined by your Detail Resolution.
The second issue is with the alphamaps on the borders between the generated slices. With a single Unity Terrain, blending on the alphamap occurs behind the scenes. Unfortunately this blending breaks when slicing the terrain, and noticeable lines appear between terrain slices. To correct this, there is an alphamap blending feature built into the slicing process. This is not the same as Unity's blending, however. Instead, all it does is set the edges of the alphamap of neighboring slices to the same value. Usually a value of 1 ("blending" only the edge values) works well and produces terrains that are indistinguishable from the source terrain, but in some instances you may notice a colored line. Try upping the blending value in these cases.
Use of a custom shader can get around this issue, though how to create such a shader is beyond this package.