Well as long as we are splitting hairs, I´m right, the term "hardware firewall" refers to a device (yes with a small OS) that is dedicated for soul purpose of firewall activities.

I disagree with your opinion that the ones on a full OS generally do a better job, while that's up for debate, the full OS has a lot more vulnerabilities which make them unfit for the job :-)

Linux based firewalls I always prefer, most hardware firewalls generally are.
And like SUSE (when you choose to install with only modules of your choosing) the OS is build with ONLY the necessary modules and component to do the job (in this case firewall).
Which leaves much less margin for error.