It might help to consider the client's particular needs. For example, if they want something with non-standard design or functionality, or the option to expand in that direction later on, squarespace would be a disadvantage because it isn't as flexible/extensible.

However, if all they want is a static boutique site, and don't want the option to ever go beyond that, then any CMS is arguably overkill.

If the client is an end user, it doesn't matter because all they see is just buttons, text and pictures.

If the client is a designer or developer, than it's a different story. In 10 years I couldn't find any better CMS package than concrete5. It has pros and cons, but as a whole - it's the best among the rest because its pros far outweigh its cons!