The principle of an MWBC is that two hots can share a neutral if they're on opposite phases, since the current on the neutral will cancel instead of adding. But isn't this only true for loads with reasonable power factors? Consider if one phase of an MWBC had a load that was nearly purely capacitive, and the other phase had a load that was nearly purely inductive. The voltages are 180 degrees out of phase, but the capacitive load's current is pushed almost 90 degrees ahead of its voltage, and the inductive load's current is pushed almost 90 degrees behind its voltage. Wouldn't this situation result in the currents adding up after all, and potentially overloading the neutral wire? Are MWBCs considered safe just because loads with such low power factors are nearly nonexistent in practice, or is there something else that prevents this from being a problem?
Even with your power factor shift it is not that large over all most motors have a power factor of 80 or better. The vast majority of loads are resistive or inductive not only in residential but industrial also, yes there are some high capacitive loads but they are rare. Harmonics actually are more of a concern for multiwire branch circuits on 3 phase systems as the harmonics can cause the neutral to become overloaded I have measured this but rarely found it to be at a hazard level.
An example of this would be 23 amps measured on a #12 neutral yes that is overloaded but there is a huge safety factor you can see this by seeing that a controls cabinet wiring 12 awg can handle 60 amps beyond the enclosure 120 amps inside (now that’s crazy) to see these values with your own eyes NEC table 430.72.B We try and limit the harmonic overloading in many cases it is only at startup when it’s the worst then it drops down to normal levels.
I have never found a problem with multiwire branch circuits being overloaded on the neutral and prior to GFCI’s being required everywhere I used to use them all the time. So yes in theory you could overload the neutral but with the safety factors built into code reality is you won’t overload split phase. Harmonic overload is very possible on 3 phase but not normally a hazard on branch circuits.