Wordpress - MariaDB as a backend database

Yes, you can use MariaDB. MariaDB is a drop-in replacement for MYSQL so compatibility is not an issue. WordPress even lists MariaDB as an option within the requirements on their website:

We recommend servers running version 7.2 or greater of PHP and MySQL version 5.6 OR MariaDB version 10.0 or greater.

Here is a resource from MariaDB on this topic https://mariadb.com/resources/blog/how-install-and-run-wordpress-mariadb.

There is no problem. It's compatible. remember you need to install php-mysql library.

Right now the answer is "yes". The problem may begin when the difference between maria and mysql will be in sql syntax level in a way which prevents having compatibility between them.

As both products do not live alone in the world and have to rely on 3rd party developer (like the developers of the php mysql module), I think that there is no chance that a major difference between the two on the "API" level will happen in the short to medium time frame. Most likely the differences between the two will continue to be in performance and admin side.