Another problem with one neutral serving two different breakers is that the neutral could be overloaded. For example, if two 15 A breakers are on the same leg of the panel, then a shared neutral would carry the sum of the currents in the two hot wires. So, for example, if 12 A was passing through one of the two breakers and 10 A through the other breaker, then the shared neutral would be carring 22 A! This would not trip either breaker and this neutral wire would get very hot and perhaps start a fire.

If two breakers are on different legs of the panel (and so the voltages and currents are 180 deg out of phase), then a shared neutral would carry the difference in the currents through the two hots and the current in the neutral would always be less than the current in one of the hot wires. Following the example above where one hot is carrying 12 A and the other 10 A, the neutral would carry only 2 A.

You should determine the voltage between the two hots in each box with a shared neutral. 240 V difference between two hots means they are on different legs. 0 V difference means they are on the same leg.

If any of your neutrals are shared between two hots on the same leg, you have a potentially dangerous situation. But even if all of your shared neutrals are serving two hots on different legs, then this is an unsafe way to wire a house.