For ring replication to work, it requires all replication sources (aka. "masters") to be online at all times. This never happens in real life, so it is just a matter of time that you get a nasty split-brain. Star replication tries to mitigate this by a crazy setup with replication filters that is painful to maintain and probably will break after some DDL statements.
Moreover, neither of these can handle conflicts. If a row gets updated on different hosts at the same time, you will get broken replication and/or split-brain.
To scale writes, sharding is usually the way to go and Vitess can be handy for that. By the way, MySQL group replication has multi-primary mode that is safer. Galera Cluster/PXC probably has something similar but I am not completely sure.
I would appreciate any comments. Specially on the upsides and downsides.