There is no risk in implementing web accessibility. Like all changes, you can expect a bit of a learning curve and some expense. And, of course, you need to make the changes with the help of qualified professionals.
Ultimately, the only real risk with regard to accessibility is in not implementing accessibility at all. A corollary to this idea is that there is some risk in postponing web accessibility work, rather than building it directly into your project at the outset. This is so simply because it is more expensive to retrofit your project with any significant effort (not just web accessibility), than it is to build as needed from the very beginning.