Is flexible gas pipe safe to use for main or long runs?
There are two types of tubing most people think of, when you say &quot;flexible gas tubing&quot;.  The first and more common, are flexible gas connectors.

These guys are typically 3-6' long, and are used to connect appliances to the gas piping. They are only to be used as a short link between the fixed piping and the appliance, and so are considered a &quot;connector&quot; and not true piping.
The other product; that may not be as well known, is Corrugated Stainless Steel Tubing (CSST).

This stuff is made up of semi-rigid corrugated stainless steel tubing surrounded by a polyethylene jacketing, and is designed and approved to be used as a gas piping method.  There may be problems caused by lightning strikes, but the solution is to simply have an Electrician install a bonding device.
The author of the linked article seems to be unaware of the differences between these two types of tubing, and is simply spreading misinformation.  While it's true that you wouldn't want to plumb your house using only flexible gas connectors, this is not what's being proposed.
If you smell gas, evacuate the building and call 911 (emergency services).