The elegance of the Zephyr BMS is a very simple circuit for each cell, while still protecting each individual cell for over- and under-voltage; allowing full protection of each cell by shutting off charging as well as discharging (throttle) before any cell reaches its danger zone - at least in theory.
There are some limitations however:
1 - If the BMS electronics or wiring fails, then this may not be detected, so the affected cell(s) are not longer protected
2 - if a cell suddenly goes to zero volts instead of gradually dropping below 3 (or 2) volts, then there is no protection and this also leads to other problems.
3 - if a pack is not charged soon after hitting the low voltage threshold, then the TC54N continues to indicate LVC, running the cells down quickly.
The problem with wiring can happen with the inter-connection between the 6s/8s/12s boards that sends the combined LVC and HVC signals from one board to the next, if a wire breaks then all the cells connected to the boards before that point will lose protection, (whether over-charge or) over-discharge will no longer be communicated for those cells. If a wire breaks between LVC output of the last board and the throttle (or brake input on the controller) then this means that the user is no longer notified of a cell reaching the lower limit by cutting drive power. If the BMS is visible to the user, then LEDs on the board may signal to the user that something is amiss, but it is not likely that a user will see a warning LED on the BMS while riding. One possibility is to connect a warning LED closer to the field of visibility, such as on the handlebars (most eBikes already have a set of indicator LEDs there) but the problem is still that even the wires connecting these LEDs can break. Also, even if the LED lights up, that does not protect the cells unless the user decides to reduce throttle.
It is possible to make a fail-safe system, but the complexity will increase much and it is not easy to test each circuit, most likely a microprocessor will need to be added to cycle through all channels and verify that the LVC signal is enabled on each channel detecting low voltage (for example by adding a resistor between the cell and the input of the TC54N plus a resistor and transistor to ground, to simulate a low cell voltage). The throttle (or brake) signal should be clamped to ground until all circuits are verified to work. Commercial BMS'es might implement this level of self-test, but it seems over the top for a DIY system.
One thing that may be done simple and easy is to add a connector to the open LVC input on the last board in the string, so that occasionally a plug can be inserted to short the signal there, then verify that the controller cannot engage due to the trottle (or brake) signal being clamped, verifying that all intermediate LVC wiring is OK.
Another even more sneaky type of wiring failure is when one of the wires between pack and BMS fails. If one of the "end" wires fails then we lose the corresponding channel, so that cell is no longer protected. If one of the intermediate cell connections fails, then the average voltage of the two cells may be OK while the actual cell voltage is too low (or too high) and the two channels might also fail to properly indicate even when the average voltage reaches LVC, because due to the votlage dividers on the 431 references, the voltages will divide perfectly between the two channels until one channel hits LVC which will then push the other channel to the HVC condition (4.2V) while the firs channel voltage suddenly drops from 3V to 1.8V. This may be too low to reliably indicate LVC, so the two cells that share a broken wire to the BMS will then be unprotected.
Then there are cases where the electronics fails, either because a component fails or something affects the BMS board (moisture, chip of metal, loose wire) and causes the fuse to blow. This will go undetected as this channel will simply stop working, so that cell is no longer protected. Also a bad solder joint or component that fails open due to thermal, mechanical or electrical shock will render the BMS inoperable without warning.
The situation that a cell suddenly goes dead will lead to two problems: since each channel of the BMS is powered by the cell that is being protected, this means that protection fails when a cell fails. The other problem is that the total voltage of the pack drops, which can lead to problems while charging, either over-heating the BMS shunt resistors or the Charge Control FET. In extreme cases the dropped pack voltage may cause the Charge Control FET to exceed its voltage rating, since it is designed to only carry the difference between charger voltage and pack voltage, but this is not likely with a 100V FET rating. It is more likely that the increased voltage difference will cause overheating once the first channel requests throttling back the charger. The danger here is that a failure will typically lead to a short circuit in the FET (typical failure mode of most semiconductors). This means a loss of control over the charger and with one (shorted) cell less in the pack, the remaining cells will get more and will be severely over-charged...
When the pack is run down to the point where the BMS disables the discharge by enabling the LVC, the low cell will continuously see the drain of the opto-coupler LED to enable LVC. Even though the 1k Ohm resistor will limit that drain to less than 2mA, the cell will see a drain that will discharge it down to about 1.2V (the opto's LED threshold) in a matter of days. So if you forget to put the pack on charge for only a few days after running it down as far as the BMS will allow you to go, you might permanently have lost the pack. It could be wise to add a disconnect when the drain should stop, or at least add a diode in line with the LVC opto to stop draining the cell at about 2V so it has a better chance of survival, even though this change will actually cause a reduction of the reliability of the LVC, because if an already well drained pack suddenly needs to dish out a large current and the cell sags instantly from 3+ to below 2V, there will be no LVC signal because the voltage is already too low....
There probably are a couple more failure modes for low cell voltage that I overlooked, but these are what popped in my head when going through the schematic of the channel and system. In case of questions/corrections: fire away!