Scaling for MariaDB / Percona / Galera Scaling
Rolando may be right, or I may be right; it depends on the situation...


A single Galera cluster (PXC or MariaDB or hand-rolled) can have more than 3 nodes, but probably should not have a lot more than that.
Writes somewhat scale with Galera.  Especially if you have complex transactions that are heavy on reads.  In that case, the transactions are spread across the nodes, hence the reads are spread around.
Slaves hanging off each node can provide arbitrary read scaling.
In WAN setups, writes are faster in certain situations.  But perhaps you are talking only about LAN?
"low memory" and "disk I/O issues" are pretty much the same with or without Galera.  Maybe you want to address them separately?
Slow reads are usually caused by too many connections, poor indexing, poor SELECT formulation, etc.  Again, this is generally independent of Galera.
Adding a node to a Galera cluster is "trivial".  (Well not quite, but much of the work is automated.)


The only real way to get write scaling is via Sharding.  There is no "automatic" sharding (aside from Spider and Clustrix), and I am suspicious about how "automatic" they can be.