Running Natural Gas Pipe to Appliances
Yes it's common but just because it's done often doesn't mean plumbers can change physics to get lengths of pipe into places they don't fit. The only thing wrong with more couplings is when black pipe leaks it's almost always at a fitting. The more fittings you have, the more possible leaks you have. So long as they tighten everything up and use dope on the threads all the couplings mean is more labor, they won't adversely affect anything.
As for gas flex, I don't trust it. It's cheaper though and more and more people just want whatever is cheapest without regard for longativity or safety so that's the way it's going.