Crypto hardware acceleration is commodity. I believe that Broadcom sold the chips for <$5 over 5 years ago. To say they don't scale horizontally doesn't make sense either. If they're integrated with PCIe then you can get a lot of crypto processing in a single chasis. (Disclosure: Used to make crypto accelerators and load balancers)

There surely are reasons not to integrate at the load balancer, but they're not because the load balancer will melt down.