Everything IMHO
Reactos has been programmed since 1996. Yes, it might be usable in the next 10 - x years and could become one of hopefully several alternatives for audio applications. Wine is not what I want. I want native (Linux audio) programs.
Windows/Mac as an operating system is becoming less and less important as it is moving to the cloud. The Windows/Mac operating system will soon only be offered as a rentable operating system. OSX has a smaller market share than e. g. the IPhone (~mobile).
With all operating systems (Mac/Win/Lin) the audio users make up perhaps 5% of the total users, thus rather an insignificant portion of the (paying) users.
I like LV2 1000 times better than VST3, but I think that Linux-VST3 can be a transition to LV2.
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Differences between the individual operating systems
Everything IMHO