Is using 10.8 (over 10.9) a security risk? Is 10.8 currently (as of 2014 Aug) fully supported, especially security-wise, or using it might be a security risk?
Not particularly at any grave security risk since Apple is supporting and releasing security updates to OS X for n-3 generations of the OS as of August 2014:


Mavericks (10.9)
Mountain Lion (10.8)
Lion (10.7)
Snow Leopard (10.6)


This is documented on the main support page at http://www.apple.com/support/osx/

I haven't seen any time when n-2 were not supported on OS X so unless you depend on enhancements to Gatekeeper or the more advanced security features that are new on 10.9 such ad Application Layer VPN, Sandboxed Plug-ins, Additional sandboxed Apple Apps, OTA certificate revocation to make up your definition of risky to stay on 10.8 then you should be equivalently secured on 10.8 for the foreseeable near future.