There are different levels of samples in terms of quality. Something like Garritan is not going to come close to more expensive libraries like Spitfire in terms of quality, and will sound cheesy etc. You can get realistic enough results with high end libraries like that, especially if you have solid mixing and orchestration chops.
NotePerformer cannot match the results of good sample libraries in a DAW. I see lots of people commenting on NotePerformer demo videos “NotePerformer sounds so real, like a real orchestra!” but if you put it alongside a mockup done with good sample libraries, the mockup will win by far every time. What this means is if that if two people do demos for scoring say a short film and one does it with notation and NotePerformer and the other with high quality sample libraries, the one who does the demo with the high quality sample libraries will win hands down every time and get the job.
In my experience, the problem with the high quality sample libraries, why you can’t just plug them in to notation software and have them sound good, is twofold. First, there are balance issues which NotePerformer automatically adjusts for but the sample libraries do not. Second, NotePerformer is able to somewhat intelligently look at the material and decide how to alter its performance based on that, while sample libraries will just march on doing what they are told. The result from plugging even a high quality sample library into a notation program will often be sub-par, unless you do a ton of manual CC work and humanization of attack. Although the sound itself may be more realistic than what NotePerformer can do, balance issues plus unsuitability of selected articulation for certain passages means that the overall result sounds a bit odd and unrealistic. Mixing different libraries together from different vendors can also be more problematic in notation software in general because the mixers are frequently less capable than those included in DAWs.
I had tried a few times in Finale and Sibelius (back when I used those) to plug third party libraries in and was very disappointed with the results, paling in comparison to the results you get by default with NotePerformer. I haven’t tried in Dorico, and I expect the new Dorico playback features can address some of that but the balance issues can still be problematic. But, for me, if I want a polished product, I’ll do it in a DAW. If I’m working in notation, I’ve usually already done a DAW mockup up or I don’t need a mockup at all, and then NotePerformer is just fine even though it can be fake sounding, since it gives a very good general idea of how things sound and the balance.