The Plan:
Start a game with one AI on a large map, challenging difficulty, cheats on. Use the view cheat to see everything. I'm going to count the number of times monsters attack his units (not the other way around) and do some stupid stuff with my own pioneers.
End Result:
I didn't get a large sample size because, after 100 turns, Magnar only built 4 pioneers.
Magnar Pioneer 1: Ended turn next to a banished ogre, but immediately built an outpost. My pioneers encountering banished ogres ended in immediate death and death after waiting one turn next to it.
Magnar Pioneer 2: Moved next to some wolves, stood there one turn, not attacked, then moved off. My pioneers encountering wolves had the same experience - they weren't attacked on the first turn.
Magnar Pioneer 3: Moved next to wolves, not attacked, then moved off of the tile but still built an outpost there.
Magnar Pioneer 4: Moved next to a wilding rider's army. Nothing happened the next turn. The next one the wilding attacked and the pioneer died. My encounters with wilding riders ended the same way - one turn standoff, then they kill me.
I had some details, but they weren't that interesting. The only other thing was that, in 100 turns, Magnar only built 4 pioneers and 2 cities (including his first city). On challenging. And the one city was founded on a 3/3 after the pioneer walked over a 3/4 to get there.
I'm going to have more time later this week, I'll give this another shot. Other people should too so we can get a real sample size.