Safe to leave oven on at 180F while at work to cook ribs?
Yes, there's a risk, but it's not significantly higher than having most other home appliances turned on  (eg, a lamp, dehumidifier, dish washer or dryer).

Although it heats up, a full-sized oven is insulated, and you're not operating at a very high temperature.  Provided it seals well, even if there's a fire, there would be little oxygen to sustain it.  I would not trust a toaster oven or desktop oven. with this sort of thing, as they're not as well insulated, nor do they tend to seal well.

As ovens are based on temperature, and will cycle on an off, rather than just feed in a constant amount of power (such as a cook top) ... so it won't get so hot that it causes other issues.

Update: I made the (possibly incorrect) assumption that this was an electric oven.  I don't know that I'd trust a gas oven the same way (as I would assume that it is not well sealed) ... although people leave their home heating systems and water heaters on without shutting them off when they leave each day, so it's probably not an issue.