Why is voltage drop unsafe?
I'll keep this short, not overly technical... and just hit the most relatable points. First, "It takes a village" of false assumptions, low quality products and even poor maintenance to cause a runaway battery overheating condition.
High voltage drop values not only reduce the amount of wattage available for the atomizer... in turn reducing performance, but can create Joule (resistive) heating, typically concentrated in a poorly designed and executed firing button.
A 510 connection, particularly an uninsulated hybrid type, can also produce the right conditions for Joule heating, but far less likely.
Battery quality of construction, along with overly optimistic, inaccurate manufacturer ratings can be contributors to failure as well.
You may need a few more "conditions" to produce an actual battery "failure". Anything from a scary but non life threatening mild venting (the reason for mod pressure vents), to runaway overheating and explosion, are a few definitions of failure.
The aforementioned
suspect quality mech mod is a prime and frequent contributor, but not an absolute necessity.
Deep sub-ohm resistance, and/or sufficiently low resistance to apply a load near or above the battery
actual MCCD. People think they're safe if the resistance is a mild 0.5Ω or so... but if an 18350 battery is used, you can still get resistive heating, and/or exceed the batteries MCCD... if the right conditions are met.
Speaking of battery ratings... these high (35, 38 & 40A) 18650 ratings are not in fact MCCD ratings, but rather pulse or "momentary" ratings. Granted, they are relatively modest pulse ratings within a time limit likely exceeding even a deep draw vapists usual application... but they are still a lie, and not the Maximum
Continuous Current Discharge. The same applies to the the smaller popular vaping batteries, where the listed amperage is above the norm.
An inaccurate read on the net atomizer resistance, where the actual resistance is lower than the measured... a poor quality Ohm meter will be the main source of inaccuracy.
For example, an assumed 0.2Ω, where the true value is less... the battery discharge can rise dramatically, not only beyond the MCCD, but beyond any reasonable pulse discharge, time to excess. Remember that battery discharge doubles as resistance is halved... so at 0.2Ω, an error of 0.1Ω means that discharge can go from a safe 21 amps to a not so safe 42 amps, in the press of a button.
If there's enough resistance in the mod when fired, independently or in combination with an extreme discharge... the localized Joule heating can also raise temperatures in the battery, causing it to fail.