WordPress is a valid question/option for them if they have a large amount of content they have to add using their marketing staff instead of more expensive developer consultants.

It also is if they don't have a years-long working relationship with you, because who is going to maintain all of this when you're gone? At least if they have to dump your work and build a new theme, they don't have to worry about figuring out a whole new data model (most of the time).

WordPress done the right way uses plugins sparingly to get the job done, outside of obvious ones like Advanced Custom Fields Pro and Post 2 Post Links.

WP isn't bad per-se, it's just the majority of people who work on it (and let's be honest, who built it) are/were rank amateurs.