As someone who speaks and understands serbian i really had huge problems with Niko. His accent sounds pretty legit, Michael Hollick did indeed a great job. But the way he speaks serbian is so freaking terrible. Everytime when Nico started to speak serbian (happened a couple of times in the storyline) it really made me cringe. He did not even try at all, i really had a hard time to understand the actual words he was trying to pronounce. I know this is not a big issue for most people who play this game, since most of them do not understand serbian at all. But its not only Niko. I think its a general problem. If they want to portrait a foreign protagonist or character they should at least hire a voice actor who can pronounce the lines he has to say decently. It influences the quality of the game a lot.
Imagine if the next protagonist will be near eastern and a american voice actor has to speak arabic sometimes . This will kill the entire legitimacy.