Skellige is rather lackluster as to how it presents itself.
On one side, people and interior details (clothes, armor, buildings, mentality) are rather close to historical analogs (I mean ancient Scandinavia) in Skellige, and everything is done in excellent graphics - it is a very significant advantage in comparison with Skyrim, for instance. On the other side, this part of the world seems to me least elaborated.
The main problem of Skellige is that it looks like a half-empty decoration, in which the witcher was placed for some unexplainable reason. Seriously, during the game, I always felt that Skellige was just a place designed to create a contrast to Velen. Geralt looks somewhat inappropriate in such circumstances.
In addition, if you pay attention to the details, you will find many shortcomings that make the world of Skellige even less substantial:
1. The design of gravestones and notice boards exactly matches those of the continent.
2. Albeit Skellige having it's own unique wide array of alcoholic brews (for instance, local beer, mead etc.), the game doesn't so much as mention them anywhere on the islands, let alone sell them through merchants, who have just the same stock as the rest of the continent.
3. Runestones and glyphs don't differ from the continental ones. Runestones and glyphs on the continent are named after the Slavic deities (Veles, Chernobog, Perun etc), thus it would be logical to assume that those on Skellige islands will be named after the characters of higher Scandinavian mythology (Odin, Thor, Loki etc), however, this is not so.
4. In addition to this, there's something else I want to comment on – monsters. This is the biggest disappointment. Where are my hulders, skogsrås, draugs, necks and ten types of trolls? I've discovered just two completely new monsters – sirens and cyclopes.
5. Even the music performed at the feast and in taverns does not differ much from that in the repertoire of Novigrad musicians.