Ps. Rainer is right: there is no magic formula, no Qt or anything else helps you with that. And each solution has its caveats.
FontLab 7 uses Qt, which is great is some aspects. Qt lets FontLab make the app affordable, because FontLab 7 can run on the Macs, and onbany inexpensive hardware that runs Windows, and even, with some limitations, on Linux. But it also makes it impossible to use some conveniences that Apple offers. And Qt isn’t bug-free.
Sometimes you cannot serve all people. Some users of FontLab 7, and I imagine Glyphs as well, on the Mac, do not want to upgrade to the newest macOS, because they use software that doesn’t run on the newest OS. For example, some people may want to still use Adobe CS6 apps for various reasons. Others actively upgrade their macOS to the newest.
But as a developer, when you use frameworks like Cocoa/XCode or Qt, you’re faced with the fact that these frameworks get newer versions which ship improvements, but often they also drop support for older OSes.
So when we build a new version of our apps, we need to choose: do we ship the app also got those who prefer older macOS, but other users don’t benefit from the improvements (or simply bugfixes) in those frameworks, or do we use the latest framework but accept that our app won’t work on older systems?
It’s not easy or sometimes not possible to do both. And the truth is, the more versions and OS combination you work on as a developer, the more complex it is, and the more testing it requires.
This is by no means limited to font editors. But font editors are particularly affected because in the last year’s, font technology has actually developed quite rapidly (color & variable fonts, also the progression of Unicode, shaping engines and screen rendering).
We all want to make type design tools that let you make all kinds of fonts, for any script and in any format. But that naturally means that we put our energy primarily into the newest versions of our apps.