There is also a size limit on the messages sent via SQS - 256Kb IIRC. Might be OK for small text based messaging, but if you want to talk attachments and other payloads then you have to hook it up to other services like S3 etc. and then it becomes complicated.

Also, as another respondent replied - there is no real deliverability guarantee, although there are certain ways to handle that within SQS.