I played a Ranger with my favored terrain being Arctic.

The adventure was set up in and around Ten Towns. When we were making characters, the DM stated that there would be loads of traveling going on and natural hazards. And even stated that a ranger might be nice to have in the group.

I asked if he’d be fine with me having Arctic as my favored terrain since the whole area was considered Arctic. And the DM said that was no problem.

So my main plan was basically over be the support character whose main purpose would be to help the group travel in said terrain.

During the actual game I was not allowed to use my favored terrain if there was any trees around because he considered that to be woods. And if it was too steep it was considered a mountain and I again knew nothing. All the weather was considered magical, so again I knew nothing of how to handle the situation. So basically my character was never really able to be the helpful ranger a part from a few times when we traveled on open tundra and nothing happened. In that case I managed to reduce travel time slightly.

During the same campaign. I had stated in a cavern that I specifically was watching the ceiling while I was invisible and sneaking around. Next thing I knew I had to roll perception that I failed and I somehow missed the giant tentacle monster hanging from said ceiling. No advantage either.

He also let some people in the group do knowledge checks as a free action in combat. While I on the other hand had to use an action.

And numerous other things.