Are there any scenarios where a neutral wire can be overloaded on a Multi Wire Branch Circuit (MWBC) when properly wired?
Also, on the same correctly wired MWBC, are there any deficits to having electronic devices as loads?
Both questions apply to outlet and split lighting circuits.
If correctly wired (to include proper size breaker and breaker properly located for 240V to the MWBC,) no way to overload the neutral. The breaker will trip.
The neutral carries maximum current when only one side of the MWBC is running a 120V load. The current reduces when the other side carries a 120V load to the difference between the loads, and if the loads are equal, the neutral current is zero (0)
The classic way to overload the neutral is to improperly rewire it so both hots are on the same phase (0 V between hots) - then the neutral will see double the rated current before a breaker trips. That is more probable on certain types of panel and likewise with old, non-handle-tied feeds, when somone ignorantly moves a breaker to the wrong phase feed. Using a common-trip two-pole breaker makes this impossible in most (all?) panels, though that is not required by code unless the MWBC feeds both 240V and 120V loads. Handle ties are (presently) required but were not in the old days (I don't know the code revision that changed on, but it's more than 40 years ago, I think.)
A properly wired Multi Wire Branch Circuit in a typical single phase 120/240v residential service overloaded neutrals or harmonics that cause overloaded neutral are not an issue.
It is rare in residential applications, but if you have a 3-phase service or 120/208v fed by two legs of a 3-phase service then the Informational Note in the section that details provisions of MWBC's NEC 210.4 is relevant:
Informational Note No. 1: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads might necessitate that the power system design allow for the possibility of high harmonic currents on the neutral conductor.
Some kinds of loads take in more energy than they're going to need during part of each AC line cycle, and then feed the excess energy back into the power mains during another part. Such loads are called "reactive loads", and can be subdivided into capacitive loads and inductive loads.
If one were using an extreme capacitive load on one leg of a multi-wire branch circuit and an extreme reactive load on the other, it would be possible for the RMS neutral current to exceed the RMS current on either leg. In general, however, the loads that are most reactive tend to operate with relatively small levels of current. An LED-based night light with a capacitive dropper circuit may have 0.05A load that is almost purely capacitive, and an electric clock motor may pose a 0.05A load that is almost purely inductive, and plugging one onto each leg of an MWBC may result in a neutral current that is greater than 0.05A. On the other hand, even if the neutral current were 50% greater than the higher leg current, that would still only represent 0.075A--nowhere near enough to pose any kind of overload risk.
If one were to plug enough capacitive-dropper night lights on one leg and enough motorized electric clocks on the other, it might be possible to make the neutral current slightly exceed 15A while keeping both legs under 15A. In practice, however, such an imbalance between capacitive and inductive loads would almost never occur in any realistic scenario, and even if it does the neutral current would be only slightly greater than the higher leg current.
To get an overloaded neutral on a properly wired MWBC would take a fault somewhere outside that circuit. The way the question is worded would require a fault outside the circuit by definition, you've defined that there is nothing wrong with the MWBC wiring.
In another question there was the issue raised of a generator that bonded the to poles of the split phase into a single phase, and using a fairly common TT-30 outlet out of a 120VAC @ 30A generator to supply power to a 240VAC @ 30A L14-30 house inlet. In theory the MWBC could produce enough current on the neutral to do some damage. In practice there's likely enough safety margin to prevent this.
Another matter is how unlikely it would be to have the entire load in the house (or nearly all of it) on that single MWBC to cause an overload before any circuit breaker trips. A generator larger than 3600 watts would be unlikely to have a 120VAC outlet rated for more current, it would use a split-phase 240VAC outlet instead.
Maybe some unusual fault occurred somewhere up to the breaker panel that could cause more current on the neutral and not trip a circuit breaker somewhere.
Are there any scenarios where a neutral wire can be overloaded on a Multi Wire Branch Circuit (MWBC) when properly wired?
Yes, there are various ways in which the neutral of a circuit with multiple hots could become overloaded. This applies to both multiwire branch circuits and potentially to distribution circuits feeding subpenls. In most circumstances they will be unlikely enough that they can be discounted, but that doesn't mean they don't exist.
