A study by SoCal gas but no firm conclusions
www.copper.org/applications/fuelgas/pdf/south_cal_gas_final_report.pdf
and a safety brochure by another gas utlitiy
www.ameren.com/AboutUs/ADC_NaturalGasSafety.pdf
it appears that corrosion in copper caused by nat gas use may or may
not be a serious problem BUT the consensus appears to be that flared
connections are WAY more susceptible to corrosion & failure than
compression fittings
Way back in the 90s I worked as an intern with a natural gas company. We were trying to bring it into a local community, and needed to get at least 100 residents to agree to sign up for it, and connect to nat gas within a year of it being brought into the community. Almost all the propane tanks were connected via copper. In all cases, our techs wouldn't hook up the nat gas until all the copper was replaced and certified by inspection, with corrosion being the primary concern. IIRC, it was black iron, but it's been a while.
I do remember one propane company in particular that knew we were coming, and didn't like it. They wound up going around to the lesser informed folks, offering them "great deals" to sign up on 18-month contracts which then prevented some folks from signing up with us due to the 1-year agreement. In all cases, after talking with mgmt about it, we agreed to waive the hook-up requirement until existing contracts expired.
Funny story, this was an old town with really poor records. Our crews ran into unidentified pipes in the ground on almost on a daily basis. Each time they had to have the city manager ID them. One day, the city manager came out and tried to ID some pipes, claimed they were old water pipes that were no longer used, and proceeded to cut through fairly new fiber-optic lines. He had the wrong maps. Thankfully it was him, not our company, that did it.
Possibly EVERYTHING one would want to know about the subject being discussed...
https://law.resource.org/pub/us/code/ibr/icc.ifgc.2012.html