There is a saying in the Drupal community, "There is a module for that". There are over 5,000 modules that can be essentially plugged into your website. If you can think it up, most likely someone else has already written a module for it. It is much more efficient for a site builder to use a tested and proven module then to write one from scratch. I know that I would rather spend my time on refining the look and feel of the website than writing a module from scratch.
The power of CCK and Views! Views in Drupal allow an advanced user to create reports, dynamic displays of data, lookups, user lists, etc. This tool is in short a smart query builder for getting access to the data on your website. CCK is a module that allows you expand the type of content that you can add to your site. For example you can use CCK to allow your members to easily upload a video or image to a page on the website. CCK integrates with Views to give you full control of the display of your pages.
A Drupal theme is what allows you to change or replace how your website looks and feels. There are thousands contributed themes for Drupal that can be easily installed. There are also several good base themes like Fusion, Zen, or Basic that make it possible for you build a website that matches a custom design. Drupal's theming system utilizes a template engine that separates HTML/CSS from PHP. Nearly all pages on your website can be overridden at the theming layer.
Drupal runs on the AMP stack - Apache, Mysql, and PHP. Nearly every hosting provider can run your Drupal website. There are prebuilt images for Drupal for Amazon EC2 and other hosting providers and there are a growing number of providers that specialize in hosting Drupal website like Acquia.
The community Drupal has a large community of active users and developers. There are over 650,000 user accounts that have been created on Drupal.org, and more than 2000 people have signed up for developer accounts. The community is growing larger each day!
Drupal has adopted a transparent policy towards security. They have a dedicated security team reviews any reported issue—whether in Drupal core or contributed module. The security team announces the nature of the vulnerability and the availability of the fixed code in a timely manner, so that site administrators can immediately take remedial action.
Ubercart is a set of modules that will allow you to sell a product, service, membership, or subscription on your website. It rivals some of the dedicated commerce software packages. Because it is written in Drupal it can be easily extended and customized.
Drupal core is modular with a system of hooks and callbacks, which are accessed internally via an API. Hooks gives a developer the power to change how a Drupal works or looks without hacking Drupal core's code. Keeping these types of modification outside of Drupal's core allows you to upgrade Drupal without breaking your custom functions.
In conclusion for a very complex website that would require an extensive set of custom modules Plone would be an excellent solution. For all other projects there is Drupal.