I raced Cartagena and I'll relate my experience although I'm not sure I'll be able to give you much insight as far as comparing the heat with other races, also I'm Colombian (although I live in the US) so that might be skewing my impressions of the race. Anyway here are my thoughts:

I loved the race, it was a great opportunity for me to experience Cartagena since I'd never managed to visit until now. If this race was closer to Xmas which is the time of year I usually come to Colombia to visit my family I would likely try to do it quite often. This was my second 70.3 after Coeur D'Alene in June and I bettered my time from that race by 25 minutes (I finished in right about 5:46 so not fast by any means and about ~100 of ~280 in my age group).

The swim is in the ocean on a bay, visibility is pretty low in the water but the course is super simple and I thought it was ok/good. One thing I found strange/annoying is that the swim start was divided by age groups rather than self seeding and my group (M30-34) was last to go which meant I had to be there over an hour and a half before starting, since transition closed at 6 and my wave didn't get going until around 7:30.

The bike course is super flat and fast and the road condition was much better than I expected from my experience with roads in Colombia, a few bumpy spots closer to the city but once you get out on the highway it's pretty nice and the scenery is quite interesting, you're riding next to the ocean but as others have said you ride through some poorer towns/neighborhoods and that makes for a unique experience. You will see tons of kids from these poorer towns out cheering and just being generally curious which I enjoyed. Aid stations stock gatorade and water and this worked out pretty well for me, specially since I would just grab a bottle and squeeze the contents out into my front hydration and then dump it.

I expected the run to be hard because of the heat. I had gone to Kona over thanksgiving to try to get a bit of heat adaptation in and also decided to wear a Desoto skin cooler top and sleeves. It turned out that I was more troubled by a knot in my stomach and tightness in my legs which took a stop at a porta potty and about 4 miles before both issues subsided to the point where I was able to run most of the remaining miles with a few walking spurts including up and down the ramps that lead up onto the wall that surrounds the city in an effort to keep from full cramping. To be honest I didn't experience the heat to be much worse than CDA, that could be because I was focused on other issues but I would say it had a lot to do with the large number of aid stations stocking ice which I stuck into the pockets of my jersey and ice cold water which I would drink a bit of and dump the rest over my head. Miraculously even though my shoes/socks got pretty soaked I had no issues with blistering. Between the aid stations and people cheering there is almost no time at all during the run where you are not being pushed on and helped by spectators which I thought was amazing, also I really enjoyed running through the walled city, it is a pretty unique experience, probably more so for me being Colombian since the architecture reflects quite well that of other towns in Colombia and gives you a sense of it's history.

As far as logistics, I would probably echo what was said before, it will be quite hard to get in or out of the walled city on race day so I would try to find a hotel that allows you to walk to and from the race. This was totally not the case for me as I was staying with a friend that lives as far away from downtown as you can while still being in Cartagena but I made it work. Taxis and cars in general in colombia tend to be super small so unless you're like me and travelling alone you will need to make special accommodations to find cars big enough to fit both bikes and people.

Let me know if you have any more specific questions and good luck with your race planning, I think you won't be disappointed if you decide to go to Cartagena.