mainly, lack of isolation and lost robustness.
In any multitenant environment,
an instance’s performance
can be affected by your neighbors. A
usage burst in a neighbor’s instance
can affect the available resources, notably
compute units and disks’ IOPS
(I/O operations per second). Your
architecture should deal with these
changes.
Also, bottlenecks might arise owing
to latency issues, even within
instances at the same datacenter.
Cloud providers offer some features
to deal with this (for example, AWS
placement groups). However, if your
architecture has servers at different
regional datacenters, you should
use other techniques (for example,
caching).