Personally, I think it is more convenient to think about things like this (ie firewall rules) as data, which makes the use of an API a convenient way to work with the data. The converse, in my mind, is that I'd have to configure each node and ensure a text representation of my firewall rules are correct. That opens the door for some thinking about concurrency that I thankfully get to avoid with an API like this.

That said, I can see your point that you are hiding some details from the OS that might be helpful such as what hosts you can talk to.

Fortunately, just because you might configure a firewall with an API rather than some Ansible plays, it doesn't mean that you can't continue to use Ansible to fill in the gaps. For example, if you did use Ansible to previously configure your iptables, you might change the playbook to call the API based on some YAML. You might use the same YAML to write some information on the host that your application can use to understand the firewall rules that are used.

The point being is that it is always good to remember these are not either/or decisions.

Lastly, I'll also speak up for those folks that don't know much about firewalls and iptables. I understand the principles, but I'm far from feeling confident managing that system myself. In my case, I'm really glad to have an option that lets me get the benefits without forcing me to operate a system I'm not well equipped to do.