It's great for certain types of apps, and solves a lot of the first 90% of your app out of the box (user management, marketing pages, etc). So you can concentrate on solving the core problem your app is trying to solve. As mentioned in other comments, you might run into scaling issues over time, but that too depends on the app.
I've built a couple dozen apps on WordPress, some of them have grown beyond MVP and are going strong. Let me know if you have any specific questions.
A good examples is Timerdoro.com which uses custom post types to manage objects and social signin for rapid on-boarding. I would not build that by hand.
Another new site is plugindirectory.info. The core of the app talks to an api and charts the results. It took me about 20 hours to build. I didn't have to create pages, user management (registration etc), curl calls, db schema, and so much else. The core of the app took a couple hours, then I just concentrated on the code that actually makes the app.
It's great for certain types of apps, and solves a lot of the first 90% of your app out of the box (user management, marketing pages, etc). So you can concentrate on solving the core problem your app is trying to solve. As mentioned in other comments, you might run into scaling issues over time, but that too depends on the app.
I've built a couple dozen apps on WordPress, some of them have grown beyond MVP and are going strong. Let me know if you have any specific questions.