Did you take a look at Oracle Goldengate?
you can replicate your tables using Oracle Goldengate and it has mongo db adapter as well.
Ofcourse, goldengate requires license...
It changes according to the case.
Refreshing Materialized views may also works, but there are lots of differences between each method.
Materialized views use dblinks.. Any failure in dblink may make you fail.
Goldengate is sophisticated tool. It can even modify the data during the replication.. It can work on heterogenous environments.. For instance, in case of Oracle, goldengate captures the changes from the redolog files..
It is related with your needs and requirements.
In case of using materialized views, you will have materialized view queries running on your source database, so according to your replication frequency and according to the data size and table definition, it may create performance issues..