Split brain scenario - RAC and PXC
I can see why Oracle RAC can handle split brain and PXC cannot.
What separates them in their architecture and data storage. In what ways ???
Oracle RAC

All RAC Instances deal with only one set of database files
Each RAC Instance has its own Log Buffer and LGWR (Log Writer) process
Writes to any Oracle RAC instance, even on nodes that go down temporarily, can have their log sequence numbers coalesced and serialized

Percona XtraDB Cluster

Each PXC node has its own complete set of data
Writes to one PXC node must be rolled back or committed on all nodes in unison
Writes to PXC nodes that are split in communication will only update/commit to its local copy of the data. Awareness of such writes by other unconnected PXC nodes does not exist when communication has been established again.

According to Percona's Frequently Asked Questions on PXC

Q: How would it handle split brain?
A: It would not handle it. The split brain is hard stop, XtraDB Cluster can’t resolve it. That’s why the minimal recommendation is to have 3 nodes. However there is possibility to allow a node to handle the traffic, option is:
wsrep_provider_options=&quot;pc.ignore_sb = yes&quot;


SUMMARY

Oracle RAC is ACID compliant as a Cluster (again, one set of database files)
Percona XtraDB Cluster abides by CAP Theorm instead of ACID compliance
See YouTube video where Vadim Tkachenko (Percona CTO) discusses the split-brain scenario and how it is your responsibility (Video starts from 17:00 mark, listen up to 19:20 or more)