Aside from the fact that I have never seen one installed inside a dwelling...............It is rated for indoor use. That being said you should keep in mind that all the things associated with burning wood will be in your house. While the Garn let's VERY little smoke escape from the door when open, you will probably experience a trace of oder if it has a good fire in it and you open the door. I would advise that you try to keep your loads on the smaller side to eliminate "puffing" when overloaded or using very dry wood. I have seen some smoke emitted from around the door under those conditions. You would also probably feel it in the house. I walked into a Garn Barn once containing two recently fired 2000's that were stuffed full of wood. The inhale exhale of the units was literally moving a 12'x14'overhead garage door back and forth in the track. I could feel the pressure change in my ears. Some extra precautions when cleaning the flue tubes will also be in order along with the need to insulate it very well. A Garn makes a very effective radiator when it's not wrapped up tight.
Aside from that, I can't think of a safer unit to have in a house. Clearances should be at least 5 feet in front for loading and 1-1/2 to 2 feet in back for access to the intake/exhaust and piping present there. Clearance on the sides can be as little as two inches from combustible material. Now, one thing to consider is access to the anode rods on the top. They are over 3' long and drop vertically into the top of the tank. Personally I don't think there would be a problem with inserting them through one of the tappings in the front of the unit but when it comes time to pull them out you'll want to get creative with a shop vac and pull a vacuum on the tank to keep the water in when you unthread the rod to inspect it. Not saying it can't be done but it will raise the level of difficulty to put it mildly. This sounds like something you really want to think through because once it's in there, you are definitely committed.