The way Gold Crowns are designed, it is impossible to assemble the skirts and rails so that everything is flush and smooth. Some of the metal will always stick out, due to the design. You can get close, but not perfect.
I hate metal on a table, particularly anywhere a cue may touch. The way I bridge on a rail, Gold Crowns scrape the shaft. I have to elevate the butt a bit to avoid that. As anyone knows, elevating the butt is not preferred. That's on GC2's and 3's. The 4's have more of a curve on the rail, so not as bad. Rail bridges on a GC2 or 3 is a no-go. Pocket liner and metal will just mangle your shaft. On the 4 and newer it is a lot better, but still not good.
Plastic pocket liners and buckets are nasty. Bridging on one and hitting a ball with a medium or firm stroke leaves a plastic streak or film on the shaft.
Leather is superior in this regard. Now, poorly treated leather can leave marks on the balls. But SO CAN PLASTIC. Worse actually. I spent lots of time cleaning plastic impact spots off the balls from the pockets of a GC. All depends on the plastic and the leather. But on average, the leather is better on the balls. It is also gentler on the cue.
There's simply nothing on a Diamond that will damage your cue, or dirty it. Also, you can use a rail bridge on any part of the entire rail surface and pockets on a Diamond and it doesn't slow down, drag, scrape or impede your stroke in anyway.
There's also no metal or the skirt assembly is such to snag or damage clothing.