I have fire mouths and Jag cichlids going on 2 yrs outside. I wouldnt call it ponds as the fire mouths are in 120 gallon pvc prefabricated "pond" and jewels in 70 gall.
I did have to bring them in for 4 weeks due to brutal cold spell .
The colors are amazing due to sun and they breed great but drawback in my case since they are not big cichlids like Oscars is that they mostly hide as I use flower pots and pvc tubes as caves,etc other than feeding .
If you enjoy breeding them and dont want to lose fry a tank is better .
The heat has never been an issue we had 2 months last yr off average 100 to 110 daily and no problem. They all get partial sun .The "ponds" are only buried up to 1 ft deep the rest of it has "sleeve".
It actually does help with keeping bit cooler . This is cory cat breeding "pond" pffffft it's 40 gallons only but the 3 pairs breed alot . It shows "sleeve" (thats what I call it lol).