>>Drupal’s reputation for being very difficult to master
It is well-deserved. It is also extremely powerful. The quip I use here all the time is that if Drupal does what you want it to do out of the box, use Wordpress."
So if you are creating a blog or brochure website, you will find Wordpress and probably Joomla so much easier.
If you are doing a custom app, the Drupal API gives you great power and the modules for Drupal are much more powerful too. There has been a trend in recent years in Drupal to create a smaller number of extremely powerful modules that let you build functionality. So the Rules module would be one example. Views would be another, though that is now part of Drupal core.
If you will be hashing structured data, there is nothing that compares to Drupal in my experience aside from a custom built system.
Would someone experienced in Joomla find there is a steep learning curve to become competent in Drupal?
Do you mean as a developer or a site admin? As developer, definitely. As a site admin, I would say not.
Also, do you mean Drupal 7 or Drupal 8? After Drupal 7 launched, the Drupal Foundation paid for some serious usability testing that informed the buidl of Drupal 8. Their goal was something easier to use than Wordpress. Unfortunately, I have yet to convert a site to D8 so I can't comment on that.
As a developer, there is also a huge, huge difference. Drupal 8 is a completely different app than Drupal 7. Huge portions of D8 were rewritten using Symfony. So if you're already a Symfony dev, you would probably find D8 easier for you than the average person coming from D7. You also would not find the learning curve that steep (another major D8 initiative was the "Proudly not invented here" initiative, meant to stop reinventing the wheel and instead using the best components out there - in theory - that already have strong dev communities, like Symfony).
In general, though, I would say that Drupal is moving away from being small-site friendly. It increasingly positions itself as an enterprise framework rather than an out-of-the-box CMS.
If you *need* a framework, and especially if you need to handle structured data, Drupal is your tool. If you don't need those things, I would go with something else.