Responsive web design needs loads of testing and the web developer needs to know what they are doing. When you consider the number of screen sizes that there are to deal with and the number of different web browsers, you can understand why.
There are so many eventualities that we need to think about too. What happens if someone adds an image that floats to the right of the text, how does that look on a mobile? If a website owner chooses to have super long titles for their blog posts, that can also affect the way things work.
Your web developer will want to make you aware of the implications of certain actions. Many people still only focus on how things look on the desktop but how will that super long text you have overlaid on your image look on a mobile? I always try to make sure that these things are taken care of in advance. Even so, it is good for you to be aware of how your layouts will translate on smaller screens.
Time and Money
A good web designer and developer will not only build a beautiful responsive website but will make sure it works everywhere. To do that effectively we need to have access to many devices and that means considerable financial investment. However the investment is definitely worth it when we deliver the perfect website.
Then there is the time element. No web developer should ever underestimate the time and investment that goes into training and keeping up with new technology. Smartphones and tablets are being upgraded and updated with increasing regularity and everyone wants the latest device. Not only that but they want their website to work perfectly upon their new device.