I think it depends on horses. My tb is dark bay and even in a uv reflective one he sweats if temp above high teens if sunny. But a few on yard are black or dark bay and are cool under same rug as iv tried.
We have horse flies, midges, mozzies & bog standard flies.....defo worth the money I found Gallop combo ones (ebay) fab value for money, made virtually same as amigo fly rugs but only cost £19.99 & my mares has lasted 3 summers
Definitely worth the money, ours have one each and are so much happier with some protection against the nasties as we're completely plagued with flies / mosquitos. Some rugs will make them sweat more than others - ours have the rambo fly buster & my mare has the premier equine 'super lite' fly rug - none of them get hot in these rugs and they do last forever which makes the cost a bit more justifiable as they don't need to replaced every year. Some of the cheaper brands will contain quite a lot of nylon which may cause horses to sweat more than usual but there are lots that don't - it's handy if you're able to go and feel the rugs in a tack shop, the more plasticy ones tend to be the ones that roast them a bit x