Regardless of low or high (>6) level, it comes down to the challenge of the encounters. One area where I think a lot of developers get trapped into the same old tropes is when they scale everything for high level characters. In ToB for example, suddenly every character you meet has >100 hit points, even the gate guards! Ridiculous. And that is probably also why all high level D&D adventures eventually end up in the Underdark, because there you can justify all sorts of high level characters walking around. I am not a fan of this kind of global level scaling as a means of creating a challenge. I liked the fact that in BG I your first level character has a chance to interact with a 15th level wizard (Thalantyr). I think I would also enjoy the converse, where perhaps my high level PC (>6) has to deal with a low level character, if the story was good!