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.
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.
I would recommend only using Wordpress for the public facing website or blog though.
Use Rails or Laravel for the actual SaaS application.
WP is difficult to work with when you start moving beyond CMS features. Also WP has ongoing security issues.
Rails and Laravel are made for building apps, is easier to work with and more secure. Also checkout Laravel Spark, it's a SaaS App in a box and can save you tons of time.
Definitely possible to build it directly in WP, but I wouldn't recommend it.
https://raison.co/building-saas-startup-wordpress/
https://www.dtelepathy.com/blog/products/wordpress-saas-appl...
https://www.npgroup.net/blog/4-reasons-you-should-reconsider...
If it revolves around:
1. Serving content
2. Selling things (on a small scale)
then wordpress would be a good platform. However if you're looking to build a SAAS that does more than that and requires custom models, interfacing with different technologies on the back end etc. then you're in for a world of hurt trying to conform wordpress. You're much better going with a framework like Laravel.
Wordpress is a good tool if your use case falls within the WP core or available plugin ecosystem.
Laracasts.com is a great place to get started learning about it.
There are millions of different ways:
You can do your landing pages in WP and easily integrate with newsletter pop-ups and all the other growth hack tricks.
If you have a SaaS you can run the whole subscription system (accounts, billing, renewals, etc.) using WooCommerce.
You can sell digital downloads of your software using WooCommerce.
You can keep your user-facing documentation in WP.
There are plugins for turning WordPress into a CRM so you can track your leads/sales.
etc.
The possibilities are literally limitless.
For my new book notification service app http://anynewbooks.com, I took a hybrid approach. The frontend is WordPress (in need of a makeover) while I wrote the backend in Rails since no plugin could replicate the functionalities I needed.
I took this approach because I wanted a proof of concept and threw it together in a matter of days. 7 years later is still standing. So I don't regret it and I think relying on WordPress is a valid option. Developers might frown upon your choice, but that should be the least of your concerns.
That said, when I finally manage to rewrite the app (I know, I know) I will likely go with an app from scratch (e.g., probably in something like Elixir/Phoenix or Clojure) in order to customize the app to my liking with no bloat from features I don't need.
Ultimately, you have this option too. You can start with WordPress, validate your idea, and if you run into limitations you can't live with, you can then switch to an app from scratch.
