I suspect the Big Mac index is not super informative in a lot of wealthier areas where the cost of shelter is growing much faster than the cost of food.
I think I'm general inflation indexes are now completely impossible to generalize because (a) inflation has been enormous in some locales and some sectors but (b) the relative price of goods is now so dependent on location.
That is, like you say, in a major city I'd expect a Big Mac to be only slightly more expensive, but I'd expect housing to be astronomical. But outside of probably like 10-15 major metros I'd expect housing to be flat if not negative in huge swaths of the US.
To be at all useful, an inflation index almost has to ignore, say, the rental cost of a 1 BR apartment in the downtown or other sought after area of a relative handful of major cities. After all, in many cases, the same dynamics often don't apply even a 60-90 minute drive away.
On the other hand, the fact that Big Macs and gallons of milk (or houses in Flint Michigan) haven't gone up much in price is more or less irrelevant to the young couple renting in Manhattan and looking for child care.
Those few major cities make up a sizeable portion of the population though, you can't just remove them from the data and call it a day. Perhaps the solution is splitting it into a "high density inflation index" and "low density inflation index".
