I'm adding my .02 on this, I work for a very large corporation 5 Billion plus, our instruction manuals 15 years ago were thin and did not include many warning, then we got taken to court for a, "Failure to Warn" notice on our instruction manual. That case cost our company millions of dollars to the homeowner that brought the case to court.
Point being:
If Mr. Buddys heaters were even the slightest bit unsafe to use in a closed condition, they would in great detail WARN that it could kill you, and if there were any and I mean the smallest chance that with the ventilation they specify that needs to be opened would cause a single fatality, they WOULD NOT EVEN MENTION IT IN THEIR MANUAL, it would strickly state that the heater has to be used outside and not inside, the small wording of ventilation need to use inside, means they know it works.
Long story short, Mr. Buddy would be out of business if even on person was killed due to faulty use by the operator, the lawers would eat the company alive .
I have been using one with ventilation in my lance 1121 with proper ventilation, and two CO monitors for YEARS, and even during sleeping. No CO monitor going off, and no problems.
To each their own. This is my opinion and experience!