My U11 got too hot for 4K this weekend, but I don't blame it - I had it in a bicycle handlebars-mounted bag on the top under clear silicone screen on direct sunlight (it may have been about 30°C that day in the shadow), 100% backlight, running Pokémon GO (heavy on GPS, data and graphics) and then I tried shooting video. Apart from this extreme situation, where my previous Xperia Z3+ would probably have restarted from the heat already, I haven't had a problem so far, my U11 is trustworthy and works great.