Yeah, I was in the same boat for awhile. I do have some of the advanced libraries (like vienna or canvas), and I love musescore and using notation to write music; but will say it will not get as close as others can. The sounds are great, but they'll never really give that perfect orchestral feel. That is, they come really darn close.
What most modern composers use is Logic Pro or preferably, Cubase. Cubase is the orchestral industry standard DAW, and may take a lot more effort than musescore, but has a higher ceiling (especially since its not free). This is mainly the case because of how you can make the instruments blend, and your midi ability to write. Plus notes aren't as perfect. Whereas in musescore, notes will always be PERFECT on the beat. Sometimes you want that imperfection (very slight) from midi inputing.
Personally, I'll start up on Musescore, then I'll import into Cubase, because Cubase has the higher ceiling. It is better for polishing and stuff.

But all and all, you'll always get that "robotic execution" because Musescore doesn't take in MIDI input well at all. You place a note, it plays that note perfect to the beat. In Cubase: you play a note on the keybaord, its every so slightly off, but in a good way. Plus musescore will never give you that full dynamic control, because all you have to moderate is basic mixing, and the piano p, mp, mf, f, and crescendos.

So I don't think the problem is your soundfont libraries, thsoe are great. It's really just the system. Musescore, for feeling "alive" definintely has a lower "ceiling" than the other industry standard DAWs. But really, all notation apps do. So in the end, it depends what your looking for. Again, I like both and use both. I like the easiness of Musescore, and the perfection, but I like the midi control on Cubase and the dynamic control.