From what I have been able to glean more mass seems to be better for CNC. There is a lot of vibration going on and you can get some weird resonance issues. And more mass seems to help combat the vibrations. The “pro” machines seem to be using larger 40mm based rails which are not just bigger but thicker walled as well. So everything is heaver.
REALLY stiff setups (all steel) can be a problem because if a steeper hits a resonate frequency the whole thing can shudder like mad. But our aluminum based builds seem to be “loose” enough to not experience this as much.
For the smaller format builds in the 500mm range a X, Y table design is often preferred. (as opposed to the gantry design) The work piece is move around and the Z axis just goes up and down. This eliminates a lot of the flex and vibration issues that come form moving the spindle back and forth.
But this increases the foot print of the machine so it is impractical for a larger build in the 1000mm or more range.
I think this is why gantry style CNC are optimized for 2.5D carving. Made for carving thinner stock and wood. Because the longer you make the Z axis the worse the flex issues get.
And X Y table style CNC’s are optimized for metal carving and have much large Z axis ranges.
Openbuilds now as a C-Beam XY Table kit. It is a CNC optimized to carve the metal plates needed to make the larger format Gantry CNC.
I find it amusing and sort of meta that there is a kit to make a machine, to carve the parts you need, to complete the kit, to make the machine you are actually trying to build… 
I don’t know how rods and linear bearings compare to other setups. But I have noticed that all the higher end machines like to use rods and linear bearings driven by a screw for the Z axis. And those who have upgraded they XC to a rod based Z axis have had very good results. (See It's Alive for more info)
It seems to me that is a more solid setup but only workable for shorter distances? Most of the rods I have seem have been around 300mm. I am not sure if this is because it is a common size used for 3d printers or if it is some kind limit of how long they can be before having problems?
I know that the acme screws, while available in longer lengths, are not used past 1000mm because they start to have problems. Something called “screw whip”.