They're fine. They're a warm bike depending on wind protection, but a Harley will be much worse in hot weather on account of the huge air cooled engine. People claim that Ducatis and KTMs give out more engine heat too, which isn't surprising considering the power they can make.
What gives off serious heat sometimes, on a modern BMW flat twin, is the catalyst. That will sometimes roast your right foot.
Forget about the silly cruiser, if you want to try a proper BMW try a GS or an RT on proper roads, like it or hate it but that's what BMW bikes are about.