[abhaga]

One of the biggest problems in building something on top of Drupal is the 2 year cycle of major releases with no backward compatibility. That means you will need to port all your custom code every 2 years. Combine that with the fact that many crucial modules are actually not part of the core distribution and are often not ported for months/years. Some less popular ones are even abandoned with new alternatives coming up. We went through that process of upgrading from D5 to D6 but simply gave up at the D6 - D7 transition.

[analog]

In my experience the work involved in porting your site to the next version is equivalent to the work involved in just rebuilding from scratch.

And this is something you have to do every 2 years if you want to be able to use any of the latest developments in web technology. It's the big dirty secret of working with a monolithic stack imo.