The risk of fire from lightning damage to CSST is extremely small. There are a couple of different ways to look at the relative risk, but they all convey the same conclusion—the risk of lightning damage to CSST and consequent fire is extremely rare.
According to NFPA statistics, there are about 370,000 fires every year in home structures in the US. (See the table for the leading causes of those fires.)
As you can see, lightning is relatively a rare cause of fire to US homes - about 1% of the total. However, gas piping fires ignited by lightning are even less common. According to NFPA statistics, there are on average about 150 fires each year in which lightning ignites natural gas. Furthermore, these types of fires have been fairly constant since 1980—about 100 to 150 per year. This means that there were fires in the gas piping system ignited by lightning before the introduction of CSST in the United States. This also means that CSST fires are only a part of the 150 lightning/gas piping fires per year. So if we assume, for the sake of argument, that half of those fires are due to CSST (75), as a proportion to the total number of fires in the U.S., CSST fires would account for about 0.02% of the residential fires in the U.S. each year. Although manufacturers are working diligently to reduce the risk of damage to their products from lightning, CSST lightning fires are very low on the scale of threats to the general public.
There are about 6 million homes in the United States with CSST installed in the building. According to the NFPA data on lightning fires, on an annual average in the United States there are about 150 fires caused by lightning that damage the gas piping system. Some of those installations are rigid pipe systems and some may be CSST. If, for the sake of argument, we assume that one-half of those 150 lightning/gas fires are to CSST systems, then the chance of having a fire in one of those houses from lightning igniting the gas piping system in any year (75) is 0.00125% (75/6,000,000). While there will always be some risk inherent with lightning, these statistics show that the risk to CSST is not significant.
Has the risk of lightning / CSST fires increased over time? The short answer is no, especially when you consider the rapid growth of housing and the installation of CSST in the U.S. Since 1990, there has been almost 900 million feet of CSST sold in the U.S., and most of that was installed in one or two family homes.
Based on NFPA statistics, the chart shows that lightning fires (PINK line) have decreased since 1993; this is also shown by the black trend line that shows a steady decrease—while at the same time CSST sales have grown significantly since 1993 (BLUE line). The green line on the bottom shows the number of lightning fires to natural gas or propane systems—a very small number compared to the total. If CSST was a significant fire hazard, the rate of lightning fires and lightning/gas fires would have dramatically increased as the rate of CSST installations grew. However, the statistics show that lightning/gas fires or lightning/CSST fires is not the threat portrayed by some.
As noted before, there were a steady number of lightning gas fires before 1990, which was when CSST was first used in the United States. The chart to the right clearly shows that there were a number of gas fires ignited by lightning in the years between 1980 and 1989, and those fires averaged about 116 per year. This was a time in which there was no CSST on the market. Since then, the number of those fires has not dramatically increased. [Note: there was a new reporting function introduced in 1999-2000 and data from that time is considered unreliable.] Furthermore, since there was an average of 116 fires per year occurring to rigid gas pipe systems, it is fair to assume that a portion of the lightning fires igniting gas occurring after 1990 continue to affect rigid gas pipe systems. Lightning fires to CSST systems are a small fraction of a small fraction, especially when you consider the other causes of residential fires listed in the first table.
As a summary, the statistics do not show a growth in lightning fires caused by CSST as compared to lightning fires before CSST was introduced into the U.S.
It should also be noted that CSST piping is designed so that it will not be affected by a fire in the premises. The CSST carrying the gas has a melting temperature in excess of 2,000°F. Some of the tests required under the ANSI LC-1 are designed to test the CSST systems to assure that there is no leakage in high temperatures that may be caused by a fire. In contrast, black iron pipe systems use pipe joint compound to seal the threads used to screw sections of pipe together, or at elbows and fittings. Many brands of pipe joint compound have melting temperatures under 500°F, leading to the risk of leaking gas from the many joints in a black iron pipe system if those joints were exposed to fire in the house—which can reach temperatures of over 800°F.