I find that trail runners are fine if it's not raining and I'm on a trail that isn't steep. I find that if I'm on trails that are so steep they're unpopular, shoes will apply uncomfortable pressure at the top of the foot when going downhill, and my toes will scruntch up. Boots are much more comfortable in that case.

Tried trail runners for a 4-day offtrail trip once. They died by the end of the trip. (I'd never seen a thumb-sized gouge in a shoesole before.) They were NB, too, which seems to be decently well built in my experience. Even if they had held up, I still wasn't happy because I kept getting twigs in them. I hear some people make up for that by wearing gaitors, but it seems like one more thing to pack.

So, I'll wear trail runners if I think I can get away with it. I've also thought of wearing ones that drain quickly when paddling so I don't have to take two sets of shoes.