I agree it's another single point of failure but i guess there is less chances of a hardware firewall failure?
That's a misconception. It's built from the same components: power supplies (failure #1), fans (failure #2), semiconductors, physical connectors, HDDs/SSDs, etc that are all subject to the same wear and tear.
The benefit of choosing those appliances will definitely be their SLAs...
In case you already have a HA virtualization environment in place, it might be worth considering virtual appliances of the bementioned and protect the single instance through the hypervisor's HA techniques. For the cost of probably added latency as the vms don't have the custom ASICs the hardware has.