For safari, I used to run into a lot of graphical bugs, (especially when pushing things into layers on the gpu with transforms). The second big category would be things that are performant on low end android phones with chrome are not performant on older iPhones and vice versa.
Semi related, I was suprised just how many bugs Safari has with animation timing while using Web Animation API. You can't possibly feature detect that, Motion one lib even disables GPU acceleration for them in Safari. In that case parsing user agent is the only option.
