We sell Drupal work, so this is the least profitable paragraph on the site and the one we would rather a client read early. Drupal is excellent at some things and expensive at others, and the cost lands on the client, not on us.
Situations where it fits
Many authors, mixed permissions, a long content backlog, a site expected to be maintained by a person who is not a developer, and a need for editorial workflow. Multilingual public sites, and anything that has to survive a change of staff.
Situations where it does not
A single-page brochure that will never be edited by anyone but its author. A product whose value is a bespoke interaction, where the CMS is a tax on every design change. A prototype that exists to answer one question in a month. A team that will maintain it is a junior developer alone with no budget for support; a system they cannot patch becomes a liability.
How to decide in a week
Name the person who updates the site in two years and what they do for a living. If the answer is an editor, the platform argument is over. If it is a developer, pick the cheapest thing that lets them move fast, and do not pay for a CMS to collect dust.