What I'm seeing is that many of these new additions like M.2 are using an existing pipeline. IOW there isn't a dedicated M2 bus. As noted above some use the PCIe bus and some use SATA-3. In some instances this robs a useful slot or two or slows that speed down because you're tapping into that bus. In essence robbing Peter to Pay Paul as they say. An advantage in one area might mean a disadvantage in another area.
M2 was developed for laptops because it's a somewhat smaller form factor. Then later it came to be used on ATX motherboards. I would agree that I'm mostly seeing M.2 as a leech on a SATA-3 bus.
I don't see any real advantage to M2 on a SATA-3 bus. There is advantage to an M2 on a PCIe bus. Raid-0 seems to be piggy backing two 520mbps lines together to get the 1000MB/sec.
M.2 atPCIe x4 speed at 2500MB/second is pretty good. Certainly an advantage, but at what cost elsewhere? You're using 4 lanes of PCIe.
Seems to be a juggeling act depending on your other requirements and motherboard design.