Chimneys on outer walls are worse on every measure compared to internal ones. They loose heat to the outdoors as you say and since they cool down a lot faster

Chimneys on outer walls are worse on every measure compared to internal ones. They loose heat to the outdoors as you say and since they cool down a lot faster than the internal chimneys it is also generally harder to get the fire going when re-lighting. The chimney on my house is on an outer wall and when it's below 0C I have to light a fire every day or it becomes too much of a hassle to get it going.