M-Lok/ARCA/KeyMod/Keyslot/etc. offer a bit more streamlined solution IF you mount stuff that uses direct attatchment to the given interface.
Example... you have an MLok handguard and you want to mount a light. Well, instead of mounting a pic rail then the light, or a light directly to a pic rail, you can use a direct MLok mount (like an Arisaka mount). This makes your handguard about .375” slimmer and therefore quicker.
Pic rails can be more abrasive on some parts of your hand while the other interfaces give the user a much smoother surface. Also, these newer interfaces and going to be lighter in weight than a comparable pic rail system because the interface is physically removing material from the handguard to create a mounting solution. Pic rails are also going to be less ergonomic because they will never be able to be a true cylindrical cross section like you see on newer interfaces because of the pic rail itself.
Now the ARCA rails that is the newest hotness for precision rifle shooting basically does the same thing, but (from what I can gather) allows the rifleman to slide accessories (bipods, tripods, barricade stops, etc) up and down the rail without detatching them. This also gives a very flat and infinitely customizable interface that can used used as imagination dictates.
Pic rail handguards do have merit, but I definitely see them falling away as a primary attatchment interface when it comes to mounting things.