F5 behind a router instead of a firewall, are there any risks??

Yep.

To my mind, the products are slightly merging in functionality. With quite a large overlap now. However I also believe that the easier task is to add the glossy firewall features to an already established product like the BigIP rather than established firewalls adding all sorts of layer7 firewall, IDS/IPS SSL offload functionality to the firewall product... F5 have expertise in the hard part of interpreting the content. While the firewall vendors may not be as up-2-date on those features.

AFM is really the management side of it. As you say, BigIP has been a deny-all by default 'firewall' for a long time now. And without good management a great product is destined to get ignored, mistreated, misunderstood... Hence my comment about if you have it...

H