* The best practices are well documented and if you follow them, you are gonna end up with good code. Good code helps you iterate faster (and with more confidence) and thats crucial when building a startup.
* There is pretty good documentation available and tons of great books that you can read. Also with every release, it gets easier to do things in Rails. Ofcourse you should have test coverage to make sure that you can upgrade rails easily. This basically boils down to the first advice about following best practices.
Basically to get most out of rails, you have to buy their philosophy and follow the best practices. If you do, you can be assured of great productivity and programmer joy :)