Vendors claiming superiority based on one single characteristic are misleading the public.
The short answer to the title question is that it is only one factor among many… and it is arguably less important than others.
This is particularly so when considering modern chemistries used today in large format lithium-ion batteries common in e-vehicles and 3 phase data center UPSs. Materials used by reputable vendors in the cathode, anode, and electrolyte are stable and appropriate for their intended application in terms of energy density, capacity, cell voltage, and so on.
Yes, among these capable chemistries one will be more or less reactive than the other, but the differences are not large.
And these differences have been mitigated by technology improvements over the last two decades and by the presence of required safety measures at the cell, module, and system level. Properly designed and implemented, these measures ensure battery failures do not lead to a fire.
With billions of rechargeable li-ion batteries sold each year, statistically these batteries have become incredibly safe and reliable. The failure rate is reported to be about 1 in 10 million cells. I don't have a number, but the percentage of those failures that result in a fire would have to be incredibly small.
Being a stored energy device, ANY battery, regardless of chemistry, has the potential of energetically failing if improperly designed, manufactured, applied, or otherwise abused.
So, while one cathode chemistry might be less reactive than another, that advantage can be erased by things like poor material sourcing, manufacturing contaminants in the electrolyte, a separator that is too thin, a poor charger design, poor cell packaging, ineffective safety measures, lack of monitoring, physical - environmental abuse, and so on.
Therefore, a battery system's safety is a product of its specific overall design and implementation. A vendor is misleading the public if they claim their battery is safer than alternatives based on a single characteristic like cathode chemistry.
So it is important to go with a good, reputable vendor who prioritizes overall battery system safety in their design and has well-controlled manufacturing processes and effective quality control systems.
You might be thinking, “but why doesn't everyone just use the least reactive chemistry” so this isn't a question anymore? Well, when you combine specific electrode - electrolyte chemistries with a specific electrical - mechanical design (voltage, cell packaging, separator thickness, electrode thickness, etc) you get a very specific set of performance characteristics and price.
This is why it's so hard to make generalizations about lithium-ion batteries. The chemistry has a substantial impact on the various performance characteristics of a battery, not just on its safety.
Manufacturers make design choices and tradeoffs amongst all these different variables to get a specific battery that meets a specific application's needs at a price that is sustainable for them.
And vendors can do this tradeoff analysis because you do not need to use the absolute least reactive chemistry in order to have a safe battery system.
Article by Patrick Donovan, Schneider Electric Data Center Blog