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.