Mistlands terrain generation varies alot.

Not sure if it was just code that they refined later on, but the first patch of mistlands I ran into wasn't "mistlands", but "mist islands". It was a whole series of completely unconnected mini-islands, with water so deep I spent 90% of my time using the longship to get around. The islands were tiny. 80% water, 20% islands.

Next patch of the biome I found had sheer mountains that were practically unclimbable. No water between them, but almost no flat ground between the mountains at all.

And the 3rd and biggest patch has huge rolling hills. Tons of huge mountains, but climbable ones. I needed to invest in food with tons of +stamina, but it was manageable. The flat areas between the mountain were mostly interconnected, and there were only small channels of water here and there to cross. And one massive channel of water separating two "continents".

They literally didn't even look like the same biome.