The customization and the ability to extend every feature, as well the integration with 3rd parties is very good.
Use of Solution:
I have used this product for 4 years.
All the changes in the platform are made backward compatible.
If you have followed the best practices you will not have a problem, however you are allowed to change the caching for different pages and that may cause a lot of problems with the performance.
All the solutions based on the platform should be implemented by people who have experience. The platform is capable of handling thousands of orders per hour and Demandware take care of the scalability and hosting infrastructure, so you do not have to worry about that.