It usually does a good job for me. One thing I've noticed is that scores that have been IMPORTED from an XML that was built using some other scoring package sometimes don't interpret properly (and playback interpretation probably was bad for the person who built the original score as well).
The reason for this is that directions and other markings might be slightly out of place. I.E. A pizzicato passage might not have the proper pizzicato sound kick in until a note or two into the passage. I.E. A slur/tie might need to be micro shifted a bit to the left and extended some to the right so the switch kicks in BEFORE the note plays and kicks out after the last note of the phrase.
The above situation is usually just a matter of first checking the dictionary to make sure the direction or articulation is defined. If so, in the case of directions (like arco, pizz, legato, mutes, dynamic markings, etc.) just move them a little to the left to insure that direction comes BEFORE the corresponding note or passage.
There are also cases where an imported score simply uses directions or articulations that aren't defined in the Sibelius interpretation dictionary. Add them

When it comes to these score interpreters...Sometimes we just don't like the way the expression maps are implemented out of the box. No problem, just tweak it as you go.
Sometimes I just have to open the 'dictionary' and tell it how I want it done. In Sibelius 7.5, I get to the 'dictionary' by clicking the 'play tab' in the ribbon and then clicking the 'dictionary' icon in the 'interpretation' group. Do NOT be afraid to make up your own little code words and alternate commands...as you can always hide them when it comes time to print.
Study these lists in the interpretation dictionary, as this is how Sibelius decides how to interpret the text, articulations, and other marks in a score or part.
Sometimes it's better not to use a sound ID change, and just do something different with CC-1 (expression volume), key velocity, or the length of the note.
If a dictionary entry does parse into 'SoundWorld' to call up a new instrument or send a key-switch...then that kind of information is going to be found in your GPO4 soundset file. On a PC, you can find these soundfiles in an invisible folder (usually C:\ProgramData\AVID\Sibelius 7.5\sounds ). AVID also offers a soundset editor that makes reading and tweaking these profiles much easier. (
http://www.sibelius.com/download/sse/main.html )
Sometimes we're just in a hurry, and don't really have time to redesign dictionaries and soundset files. Here are a few tips on 'quick fixes' right in the score itself.
One approach is to not use a sound-set at all, use non key-switched versions of patches that you dial up, manage, and save in the ARIA player itself, each on their own channel. Use the Play/Interpretation Dictionary to build things you need on the fly. If that's not enough, and you need to work across several different channels for a single part, then make a separate staff for each articulation or direction of a part underneath your main staff (the one that will be printed). When you need to move to a different sound, just copy and paste to the appropriate staff and use the inspector to mute those notes on the main staff for that part (the one that gets printed). Hide what you don't need when it's time to print.
Another approach is to not use a soundset at all, and just enter key-switches and controllers via text on the staff. If you begin text with ~ it'll automatically be hidden, so it's easy to drop in specific MIDI data right on the score.
I.E.
~C68,127
That would send cc68 on to the instrument (In GPO's case....legato).
~C68,0
That would release the legato switch.
You could also combine it with unhidden text like this:
Legato~C68,127
If you need to send a keyswitch you could use:
~N#,64~O#,64 (Where # is the MIDI note number you wish to send)
To get a better handle on how to quickly and easily drop midi messages into a score...see the Sibelius manual (MIDI messages).
Hope this is helpful...a quick summary....
I once spent a day or so trying to make universal expression maps for all my synths and samplers (both hardware and software). More of my sound engines do NOT have soundsets included for Sibelius than do. Well, it didn't take me long to realize that every score can need something quite different, it's really hard to predict and plan for all this in advance, so I was always needing to tweak stuff anyway....so now I just build it on the fly, and occasionally go back and add stuff that I 'use often' to my main 'soundworld/soundset' files.
I usually work in this order:
1. If it won't interpret properly....make sure it's positioned before the note/passage in question (in the case of text directions, dynamics, some types of slurs and lines, etc.), or properly attached to note(s) (in the case of articulations).
2. Check the dictionary. If it's there but wrong...fix it. If not there, add it.
3. Just use hidden text to send MIDI messages.
4. If I'm trying to do something that needs more than one MIDI channel, such as mixing and matching sounds from different VSTs or synths in a single part...just use multiple staves and 'mute/hide' accordingly for proper play and printing.
If I really need a super nice mock-up/recording....I export it all as a MIDI file, load it into a DAW...and go to work there, where specialized editors are available for easily getting at anything and everything MIDI.