Downtown Portland has definitely gone downhill quite a bit in the last 5 years. I live in SF and lived in NYC before so I'm plenty used to homelessness and drugged-out zombies on the streets. It's not as bad as certain areas of SF, but it's certainly heading in that direction. A lot of people, especially those from more suburban areas, are not used to seeing things like this, so people are giving warnings with good intentions. Just a few weeks ago I walked around downtown Portland and saw one person actively breaking into a car and another drugged out homeless person aggressively follow us for a few blocks after asking us for money.

That said, I think 10th and Yarnhill isn't too bad. Avoid the Old Town/Chinatown area.