TL;DR: is a ventless gas fireplace safe for occasional use?

I have a somewhat small fireplace in a 1920s house in New England. A chimney expert has recommended that we not burn wood due to the poor design of the chimney interior (he thinks we'll get significant smoke back in the house). I am interested in a gas insert for two reasons: occasional ambiance and a reliable heat source during a power outage. Our main heat source is gas-fired baseboard radiators, which requires power to operate. We've already got a gas line in the basement below the firebox, so it wouldn't take much work to hook it up (I've had a plumber take a look).

My main question is about vented vs ventless. When we first looked into it, I was gobsmacked by the cost of a vented unit, and we just gave up on the idea. Then, after realizing that all of my family members have ventless units and are happy with them, I decided to give ventless another look. My main reason for considering ventless is the cost of the unit and the avoided cost of running a new chimney liner.

When you read up on ventless, the opinions tend to fall into two categories: 1) it will make you sick and ruin your house, or 2) I've had one for years and I love it! Can anyone explain why the opinion is so dramatically split on this? Is it a matter of new vs old units? Is there any particular technology I should seek or avoid? Are the dangers overblown if it's a unit that's used only occasionally and not as the main heat source? I'm quite confused.