Scaling MySQL will be your bottleneck, alternating that with optimizing queries and database usage in third-party modules. Most of Drupal Core is written well enough.
Third-party modules support using memcached, mongodb, etc. These mostly help with anonymous, session-less traffic.
Scaling out your authenticated, logged-in users means almost no caching, requiring more web servers and more MySQL scaling, unless you architect your application to support caching page segments updated with JavaScript from the start.
Some of the work to support Edge Side Includes that's going on, and whatever became of the "Butler" initiative, are improving that work for authenticated users.
Like any system, it has its upsides and its downsides.
I would add one comment: most projects fail because they don't get written, and they never get to the stage where scaling matters. Thus, I would focus on picking a tool that will allow you to get your project to the minimum viable product stage as cheaply as possible.
If you are starting a new project then I think you'll enjoy picking something more modern to use as your foundation.