Global plans charge a blended rate for the freedom to land anywhere. That freedom prices in the expensive countries, so a global gigabyte always costs more than a Thailand gigabyte, and our own Price Index makes the spread concrete: data averages $0.86 per gigabyte in Europe, $1.63 in Asia, $6.00 in the Caribbean, with Thailand at $0.22 against Monaco at $17.52. One blended price across that spread is convenient, and convenience is exactly what it costs.
So the comparison the nomads recommend is the right one: for any planned trip, price the country plan and the regional plan first, both two clicks away on this site and both quoting the same live database as this page. The global plan should win only when your itinerary refuses to be known in advance.