The cost of shipping a preconfigured Cisco firewall to some location, finding an appropriate place to put it, configuring it and maintaining it will be orders of magnitude higher in all respects than a bespoke device. And it still may not support the TCP-level proxy features that it really should have unless that’s all offloaded to some remote concentrator, which makes for either a not-so-fun HA configuration or an unfortunate single point of failure for access to the entire fleet. UniFi won’t be any better (you’ll be stuck setting up each node as its own “site” and you’ll be stuck with maintaining a bunch of sites on the ever-changing UI platform).