FScene X provides enhanced replacements for a large proportion of these default textures, to give a much more 3-dimensional visual effect created using shades that are much more realistic for their particular location in the world.
FScene X does not affect the use of add-ons for roads, rivers, traffic, clouds and autogen, since it replaces only the ground textures in Flight Simulator. - FScene X works perfectly with add-on mesh terrain scenery since it replaces the ground textures in Flight Simulator and, just as the default textures will flow to match the ups & downs of the terrain, so will FScene X.
FScene X works perfectly with custom airport sceneries - it will only affect the ground outside the perimeter of airports. All the default airports & any custom ones you have installed will appear exactly as intended.
FScene X is carefully designed to work with the default autogen in Flight Simulator. It also has the added advantage that its enhanced 3D appearance can give the ground a much more 3D appearance even from altitudes where the autogen is no longer visible.
FScene X will not slow down the performance of Flight Simulator. Some of the replacement textures for FSX are even smaller than the default ground textures. You are therefore likely to see an increased level of performance in some areas after installing FScene X.