I have two small ones and one big buddy, they see extensive use in my fish house (ice fishing) and during deer season I have a insulated enclosed deer stand on my property.
Other than melting a few things that brushed up against them I have never had any problems, but I would hesitate using too long in a very tightly sealed space without a little ventilation.
They are pretty much the standard in this area for icefishing so if there were any major problems with them I would have heard of it by now