A potential client has these concerns about using DotNetNuke as a framework to rebuild their current web site. I know DNN is one of the best CMS tools available and is super flexible as far as functionality is concerned but how do you tell them that to relieve their concerns?

Thanks for any comments!

As far as functionality - As mentioned, DNN is a framework, which allows for, pretty much, any type of functionality your client needs. All it takes is a module [or set of modules] that performs that functionality. Custom development can take care of any functionality that existing modules don't provide.

DNN for me, is the easiest CMS's I've used to admin. First time I used it, I understood how to do things in it. I haven't had the same level of ease that I did with DNN. 

Custom modules can be bought/created to simplify any administration functionality that needs to be made simpler. More cost, but it can simplify any tasks.

DNN's functionality is only limited by the imagination of module creators. :)

There is the Professional version of DNN which might make the client feel better about using it knowing there is support backing behind it. 

These are just my opinions and thoughts - but I hope they help.