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.

I have heard so many different conflicting things on this topic. Our basement used to have a kitchenette in it but it was ripped out by the previous owners. They did however leave the natural gas hookup as well as vent piping exhaust for the hood that used to be over the stove. The basement is our family room and it can get chilly in the winter so I have always wanted a fireplace...but I also don't want to poison my family. Any advice on the topic? Any of you fine fellow homeowners have vent free fireplaces in your house?