Ghost is not necessarily incompatible, but MariaDB is not supported. No one to my knowledge is maintaining a list of what breaking changes there might be and the Ghost devs aren’t testing with MariaDB before releases.
If you’d like to get some reasonable confidence if MariaDB is compatible today, you could modify the test suite or C/I pipeline code to run all the tests against MariaDB instead of MySQL. If they pass, there’s a reasonable chance that Ghost is compatible with MariaDB today. But there’s no guarantee that will hold true tomorrow, next week or next year. Each time before you upgrade Ghost with your MariaDB, you should re-run the test suite and see if there any failures. You could potentially automate runs of the test suite against MariaDB to continually check.
There was also a related thread on the topic a few days ago: Ghost 5 + MySQL 8, what specific functionality?
There are clearly some other people interested to know the exact compatibility issues if you’d like to investigate and start maintaining a list and automate compatibility checks.