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There are three issues here:

• The quality of the food
• The safety of the food
• The safety of your home

Food Quality

From a palatability perspective, you may or may not get a decent result, depending on how high the internal temperature of the ribs rises.

180 F is very close to the temperature that needs to be achieved to effectively convert gelatin into collagen and create the tender texture out of the tough ribs as any kind of reasonable rate. If the temperature inside the ribs doesn't rise that high--and air is a poor transmitter of heat, which is why you can stick your hand inside a 500 F oven--you may not get a great result.

Food Safety

The same issue--temperature--is key for safety.

You want the ribs above 140 F as quickly as reasonably possible to inhibit pathogen growth. It is not clear that the ribs will get that hot in such a low oven, or if they do so, if they will do it in a reasonable period of time (less than say, an hour) so you may have a potential food safety issue.

This is exacerbated by the fact that most ovens have a considerable margin of error between the set temperature and the actual temperature (which also varies above and below the set point).

Home Safety

Carey Gregory indicates in the comments, based on his 15 years in a fire department, that leaving the oven on is not a significant risk.

There are three issues here:

• The quality of the food
• The safety of the food
• The safety of your home

Food Quality

From a palatability perspective, you may or may not get a decent result, depending on how high the internal temperature of the ribs rises.

180 F is very close to the temperature that needs to be achieved to effectively convert gelatin into collagen and create the tender texture out of the tough ribs as any kind of reasonable rate. If the temperature inside the ribs doesn't rise that high--and air is a poor transmitter of heat, which is why you can stick your hand inside a 500 F oven--you may not get a great result.

Food Safety

The same issue--temperature--is key for safety.

You want the ribs above 140 F as quickly as reasonably possible to inhibit pathogen growth. It is not clear that the ribs will get that hot in such a low oven, or if they do so, if they will do it in a reasonable period of time (less than say, an hour) so you may have a potential food safety issue.

This is exacerbated by the fact that most ovens have a considerable margin of error between the set temperature and the actual temperature (which also varies above and below the set point).

Home Safety

RI Swamp Yankee has located a reference to the US Fire Administration which clearly recommends not leaving cooking appliances unattended when no one is home:

The leading cause of fires in the kitchen is unattended cooking.

• Stay in the kitchen when you are frying, grilling, or broiling food. If you leave the kitchen for even a short period of time, turn off the stove.
• If you are simmering, baking, roasting, or boiling food, check it regularly, remain in the home while food is cooking, and use a timer to remind you that you're cooking.
• Stay alert! To prevent cooking fires, you have to be alert. You won't be if you are sleepy, have been drinking alcohol, or have taken medicine that makes you drowsy.

There are three issues here:

• The quality of the food
• The safety of the food
• The safety of your home

Food Quality

From a palatability perspective, you may or may not get a decent result, depending on how high the internal temperature of the ribs rises.

180 F is very close to the temperature that needs to be achieved to effectively convert gelatin into collagen and create the tender texture out of the tough ribs as any kind of reasonable rate. If the temperature inside the ribs doesn't rise that high--and air is a poor transmitter of heat, which is why you can stick your hand inside a 500 F oven--you may not get a great result.

Food Safety

The same issue--temperature--is key for safety.

You want the ribs above 140 F as quickly as reasonably possible to inhibit pathogen growth. It is not clear that the ribs will get that hot in such a low oven, or if they do so, if they will do it in a reasonable period of time (less than say, an hour) so you may have a potential food safety issue.

This is exacerbated by the fact that most ovens have a considerable margin of error between the set temperature and the actual temperature (which also varies above and below the set point).

Home Safety

This is a cooking site, so I won't berate this issue, but are you willing to take the risk of operating an oven when you are not home to react to any emergencies which arise?

There are three issues here:

• The quality of the food
• The safety of the food
• The safety of your home

Food Quality

From a palatability perspective, you may or may not get a decent result, depending on how high the internal temperature of the ribs rises.

180 F is very close to the temperature that needs to be achieved to effectively convert gelatin into collagen and create the tender texture out of the tough ribs as any kind of reasonable rate. If the temperature inside the ribs doesn't rise that high--and air is a poor transmitter of heat, which is why you can stick your hand inside a 500 F oven--you may not get a great result.

Food Safety

The same issue--temperature--is key for safety.

You want the ribs above 140 F as quickly as reasonably possible to inhibit pathogen growth. It is not clear that the ribs will get that hot in such a low oven, or if they do so, if they will do it in a reasonable period of time (less than say, an hour) so you may have a potential food safety issue.

This is exacerbated by the fact that most ovens have a considerable margin of error between the set temperature and the actual temperature (which also varies above and below the set point).

Home Safety

Carey Gregory indicates in the comments, based on his 15 years in a fire department, that leaving the oven on is not a significant risk.

In terms of the food, you are fine, except that you don't have any ability to monitor their progress.

It is never a good idea to operate a cooking appliance (other than a crock pot or slow cooker) when no one is in the house, due to the potential fire hazard. This risk is small, but...

There are three issues here:

• The quality of the food
• The safety of the food
• The safety of your home

Food Quality

From a palatability perspective, you may or may not get a decent result, depending on how high the internal temperature of the ribs rises.

180 F is very close to the temperature that needs to be achieved to effectively convert gelatin into collagen and create the tender texture out of the tough ribs as any kind of reasonable rate. If the temperature inside the ribs doesn't rise that high--and air is a poor transmitter of heat, which is why you can stick your hand inside a 500 F oven--you may not get a great result.

Food Safety

The same issue--temperature--is key for safety.

You want the ribs above 140 F as quickly as reasonably possible to inhibit pathogen growth. It is not clear that the ribs will get that hot in such a low oven, or if they do so, if they will do it in a reasonable period of time (less than say, an hour) so you may have a potential food safety issue.

This is exacerbated by the fact that most ovens have a considerable margin of error between the set temperature and the actual temperature (which also varies above and below the set point).

Home Safety

This is a cooking site, so I won't berate this issue, but are you willing to take the risk of operating an oven when you are not home to react to any emergencies which arise?