I hunted buffalo in Zimbabwe in the area just across the river from Kruger in February with my father in 2012. Due to work considerations, that was the only time we could get enough time off to go. We were both successful and took two very nice buffalo trophies, but I would not recommend going that time of year if you can help it.
The outfitter made expectations clear to us before the hunt that it was going to be very hot, humid, and was going to rain several times over the course of the hunt. While we were there, temperatures varied between 85-113 degrees Fahrenheit.
One advantage of going that time of year is that we were the first clients of that hunting year, so none of the animals had experienced any hunting pressure to speak of in the previous few months. The outfitter worked hard for us and we were ultimately very successful, we just had to work our tails off.
It is absolutely possible to have a great safari like we did by going on a very early or very late season hunt, but just make sure you know what you are getting into and can handle the conditions.