I've never had a problem with exact temperatures on brooders OR on incubators. When I first read about incubating eggs, everything told me to keep it at the exact temperature of 99.5 degrees, or 102 degrees for smaller incubators. But the temperature fluctuation in my sister's house is bad. So I just kept it between 99 and 102 as best as I could, and my hatch rate is almost 100%.
As for the brooder, I don't pay attention to temperature at all. I use an old giant rabbit cage that is about 3 feet long, 20 inches deep, and about 18 inches tall. I like it because it has a wire door on TOP for easy access. But all I do is clip a red heat lamp to the front to light up only one end. I put the food and water on the far end so they don't get hot. The chicks are allowed to walk into the heat or out of it as they please. How far out they are, determines how warm or cool it is.
Once they either need a cage cleaning, or get too loud, I move them to an outdoor rabbit hutch that is closer to 6 feet long, 30 inches deep, and 2 feet tall. The bottom is made of wire, and it stands about 2 feet off of the ground. There is another heat lamp out there, but it sits on top, and lights just a single corner on the "private" end (sides are wood, but bottom is still wire). Outside of the heat lamp, they are completely at the mercy of the weather. I've had some out there when temperatures fell below 32 degrees, and they were barely a week old. I've got some now with temps in the 70's PLUS a heat lamp, and they're still wanting to rest under it. Down in Georgia, the weather can hit both extremes within a year, plus be drenched one month, and a bad drought the next. So they really do get subjected to a lot of extremes here.
The only time I have lost a chick from weather, was when my nieces forgot to put the top on the outside brooder for 2.5 week old bantam chicks. It stormed REALLY bad that night, and the temperature also dropped. The combination of cold and wet on unfeathered chicks killed one, and almost killed the other two. They didn't get under the heat lamp then, either. But as long as they have been dry, there have been no problems.