Is the Vivid brightness app safe to use?

Yes, Vivid is safe. It enables the extra brightness range macOS already reserves for HDR video using standard Apple frameworks, not low-level hacks. Your panel stays inside the limits Apple drives it to anyway. TechRadar and Digital Trends have both covered it, and macOS still throttles brightness if the screen runs hot. MacBrightness works the same way, with no noticeable CPU overhead. The only real tradeoff for any brightness booster is faster battery drain and a panel running about 5-10% warmer, which macOS caps for you. Same effect, no subscription, no account. MacBrightness is $5 one-time, and its free Splitscreen Mode lets you test the brighter screen first.