This article is biased to selling you more F5 equipment but is a reasonable summary:
https://www.f5.com/resources/white-papers/the-myth-of-network-address-translation-as-security
Long story short is that NAT is eggshell security and you should be relying on actual firewall rules (I wouldn’t recommend F5) instead of the implicit but not very good protections of NAT.
What would you recommend? I have a client with some pretty old hardware (FVS 318) installed that I suspect is causing some issues on their network.
Honestly, these days I have no idea. When I said “wouldn’t recommend” that wasn’t an assertion to avoid; just a lack of opinion. Most of my recent experience is with Cloud vendors wherein the problem domain is quite different.
I’ve had experience with most of the big vendors and they’ve all had quirks etc. that you just have to deal with. Fundamentally it’ll come down to a combination of price, support requirements, and internal competence with the kit. (Don’t undermine the last item; it’s far better if you can fix problems yourself.)
Personally I’d actually argue that most corporates could get by with a GNU/Linux VM (or two) for most of their routing and firewalling and it would absolutely be good enough; functionally you can do the same and more. That’s not to say dedicated machines for the task aren’t valuable but I’d say it’s the exception rather than rule that you need ASICs and the like.