I've worked on many Drupal sites, and I would say the sweet spot is a client who is going to budget $10k+.
Basically, under $10k budget means you should be looking at simpler, pre-baked solutions, like Wordpress, Squarespace, Shopify, Wix, static HTML, etc..
I've worked on Drupal projects from $25-250k+, and it can handle pretty much everything (sometimes requiring a lot of TLC tho).
There are so many contributed modules that can help build things out, but I always need some type of customizations requiring (usually a few) custom modules per project. It all depends... I did one project that had dozens of crazy features that we broke out into 70+ custom modules...
But yea, I would say the sweet spot is a customer who needs a pretty large site with a lot of functionality. My 2 cents anyway.
This is some very useful insight, thanks! I've been asking this since at times, fully custom development (I ship Rails apps mostly, since 2005) is not the way to go, so I'm looking for other options where using modules (either custom or purchased) could help.
I would like to make a point that a very nice thing about the Drupal community is a lack of paid modules. There is a conscious effort to either open source or custom develop. There are not very many "paid extensions." The community seems a lot more focused on creating low level tools that you can use to build a finished product.
This is in part due to the relative complexity of setup. You do not get your grandma performing a one click install of Drupal and then spending $40 on a theme and buying an extension or two from a marketplace.
The focus on earning money with Drupal really seems to be in being a good developer and knowing how to use the thousands of opensource tools that are available to Drupal. You do not see people cranking out apps or modules for drupal and selling them in a marketplace. This really sets it apart from it's php cousins Magento and Wordpress.
I would recommend for larger-type websites, and not so much for performance-based applications. However, ignoring my own advice, we did build a todo list app powered by Drupal & React.js and it works pretty well: http://www.135list.com
You can literally build anything in Drupal, although I'm not sure you should...
I would suggest you consider drupal as a basis for your fully custom sites. You start out of the box with a well supported, standards based system for the basics: user management/authentication, data modeling/storage/query, routing, path handling, HTML templating etc etc. It has a huge ecosystem of modules, also effectively out of the box, for third party integrations and all sorts of esoteric requirements. So you start writing your custom code on an established base with a strong API for modification and integration, which means less custom code to support.
In a Drupal custom module you can do or change literally anything, and you can write whatever architecture you like for your individual component. You just have to integrate with the Symfony2 router and dependency injection container. Pretty cool stuff.
Unfortunately, the book I usually point people to [1] when they ask a question like that is not yet updated for Drupal 8. Frankly, we're probably going to be in that awkward state where supply for learning resources for D8 is greatly overwhelmed by demand for a little while at least. But it's OSS; if all else fails, you can trace the source code, assuming you've got the PHP chops.
There are so many contributed modules that can help build things out, but I always need some type of customizations requiring (usually a few) custom modules per project. It all depends... I did one project that had dozens of crazy features that we broke out into 70+ custom modules...
But yea, I would say the sweet spot is a customer who needs a pretty large site with a lot of functionality. My 2 cents anyway.