Midwit take is to say that SSBNs or truck launchers are better than the Metal Gear, SSBNs can be followed from the port when they need to rotate crew and resupply food etc. and them tracked by undewater sensors and truck launchers are tied to the large main roads in the country and thus can be easily spotted and tracked by recon satellites.
Metal Gears are much more agile and can go to any terrain so it's easier to avoid detection and tracking, the fear is that Nations are going to lose track of each other's arsenal which will inevitably lead to more tension and more intense nuclear arms race.