Is it advisable to use a beta version of unity(2019.3b) to make an actual complete project? What are the disadvantages?
The downside of using a Beta version is that it might contain bugs that are never fixed in that version, or features implemented in a preliminary way that end up being implemented differently or scrapped altogether in a future stable version.
If that happens, you might find your project is dependent on a version of the engine that is not stable, or lacks the full feature set you want, but you can't easily upgrade it to the fixed/complete version because the API changed between the Beta and the stable release. This leaves you with a lot of refactoring to do, or a complete port over to the stable version.
So, it's generally considered better practice to use the main line releases, or even a Long-Term Support version, when starting a new project. That way you have confidence that the version you're using has been thoroughly tested, that bug fixes will continue to be available for it, and if you need to do an incremental upgrade later in your project, major refactoring should be more rare.
Beta versions are useful to get a preview of new features, to evaluate them and offer feedback to the devs, or to begin updating tools/middleware you've made so it's ready to share with developers using the new features in the next stable version when it releases.