Comment 30 for bug 560795

IMHO heuristics are doomed to fail badly sooner or later. The simple path is wishful thinking

Complex font features are not limited to specific locales such as Serbian, Serbian is just a locale that needs it more than others. Complex features adoption start there and then spreads elsewhere (some people thought unicode was not necessary for western european locale support, and then German speakers standardized upercase ss, oops)

It's much safer to always use the complex codepath, instead of betting on some locales never using tech that is available anyway due to other locale needs. Now that the tools to create complex fonts are there, someone will find a way to use them even for the 'simplest' locales.