Watchhentai.net is a well-trafficked adult streaming site, but being popular doesn't automatically mean it's transparent. Legitimate adult sites typically lay out clear terms of service and a privacy policy, especially when they offer user accounts. Watchhentai.net doesn't have either. That's a red flag for anyone thinking about registering.
The site itself has been around in some form for years and uses solid hosting infrastructure, so it's not a random scam. However, the lack of an about page or any hint of who operates it means you're trusting an anonymous entity with your data. If you do decide to use it, consider using a throwaway email and avoid sharing any personal information until you see proper legal pages. For a site with this much traffic, they should know better.