One of the unique things about NotePerformer is it’s ability to do more than just trigger sounds based on notes. Every library does that. According to Arne best I understand it NP reads ahead, looking at combinations of notes with slurs, articulations, etc., and actually interprets how best to implement them.
This is the key feature of NP. VST instruments require the user to either control all aspects of playback with Continuous Controllers or create detailed Expression Maps, often a combination of the two. There’s a LOT of extra work beyond the score itself. With NP all this work is taken off the shoulders of the composer.
Because the full scope of NP’s ambition is truly beyond the limits of any currently available software, it is not perfect, but it is the best I know of at what it does. The quality of instrument sounds and playback rendering is not at the level of a high-quality virtual instrument with skilled controller input, but the quality instrument libraries are very expensive, and it can take years to get good at controller input, which also required DAW software that is another expense.