Electrician for 43 years here. Using common neutral for multiple circuits was and still is legal. Residential 120-240 3 wire (Black Red White) network and Commercial 4 wire 3 phase (Black Red Blue White ) network is how almost all wiring was done unless specifically specked otherwise. Each wire was connected to it's own single pole breaker on separate phases. 3 Phase network was 3 110V circuits on a single neutral, and single phase was 2 120V circuits on a single neutral. We use all metal conduit around here so the ground was provided by the conduit. Recent changes to the code has made this type of wiring not as popular. Almost every circuit in residential now is either Ground Fault or Arc-Fault and they require their own neutral. New requirements to lock the breakers together so if one circuit blows the other circuits on the network will also de-energize. If you do use a network you have to identify the neutral for it throughout the wiring path. It is much easier now to just run a bigger pipe and have each circuit have its own neutral. There is nothing wrong and it's perfectly safe to use a common neutral wiring method unless some handy dandy do it youselfer opens a box with a couple neutrals in it and wires a couple of circuits from the same phase on to a neutral, thus creating an unbalanced load and turning the wire into a heating element!! It is still critical to keep the circuits identified throughout the run. Started using white wires with colored tracers the last few years I worked to keep everything straight. Sorry for the long winded response.