Well, the perks is that they can mine faster, find more ores via digging underground, more mineshafts and increased chance of finding diamond.
The downside is that there are no trees, no dirt on the land, no grass, no water, phantom spawns on the second night of not sleeping instead of third, increased hostile mob spawning and the only food in the region can be found in mineshaft chests (may include seeds, dirt, god apples, apples, bread, etc.).
May be a bunch of downsides but that will make Mountaineers more balanced. If this list didn't exist, more people would be Mountaineers and go chaotic since the area would be so OP. But hey, at least I made it stupidly strict.