I don't know about fire risk, but since you specifically mention spacecraft, it's probably worth keeping in mind that with any kind of reaction drive in space, a major consideration will be to reduce mass. If you can get away with even just a 1% reduction in the mass of the spacecraft hull, you're going to do that because that's less mass that you're going to have to push around using the engines (whether a large main engine or a small thruster) which itself requires fuel and reaction mass that you had to bring with you. A high pressure atmosphere, regardless of composition, will require a stronger hull for the pressure vessel, and I have a very hard time imagining a situation in which "stronger" does not equal "more mass" (if you don't need as strong, then you can get away with a less thick pressure vessel skin of the same material). So even before you get to the issue of the mass of the interior atmospheric gases, you're already adding mass to support the higher interior pressure.

That's a very good point, not least because spacecraft designers would probably be able to choose non-flammable materials for most of the contents (although that hasn't always been done). Still I'd be interested to know if it would work in a less mass-constrained setting.