1. You get to know the product in and out totally. you will know what features are on the roadmap, what can be improved, what will work for the future, what wont, etc.
2. you know all the design styles, you maintain a perfect style guide, you stick to the rules for all new features.
3. Every design you do, will 'suit/fit' the existing layouts and screen. Nothing looks new.
4. Burn out is natural.