The network is not reliable, but usually the cost of manually fixing problems arising from infrequent types of instability is less than the cost of pre-emptively addressing the issue.
As a practical example, our preferred HA solution for MySQL replication has effectively no network partition safety - if a network becomes partitioned, we'll end up with split brain. However, we have not once had to deal with this specific problem in our years of operation on hundreds of servers.
That said, do make the assumption that your AWS instances will be unable to reach each other for 10+ seconds on a frequent basis. Your life will be happier if you've already planned for that.
As a practical example, our preferred HA solution for MySQL replication has effectively no network partition safety - if a network becomes partitioned, we'll end up with split brain. However, we have not once had to deal with this specific problem in our years of operation on hundreds of servers.
That said, do make the assumption that your AWS instances will be unable to reach each other for 10+ seconds on a frequent basis. Your life will be happier if you've already planned for that.