Hiya,
FusionAuth is only supported on mysql and postgresql. People have made it work on mariadb, but I think it'd be a large effort to port it to a nosql database like mongodb. There are no plans on our side to do so.
The system uses mybatis and I don't see any support for mongo at the mybatis website: https://mybatis.org/mybatis-3/
So, I'd say it is far beyond "extra tweaking" to make FusionAuth work with mongo.