What you don't mention, and what is important generally for these kind of questions, is specifically what kind of hardware you have, what your requirements are (throughput, concurrent connections, cryptography load, etc.). What's a snapshot of your traffic look like, where's it coming from, and what, ideally, should be going in and out?
(And you've also got to worry about worse case scenarios -- depending on who you are supporting -- the world, external customers with SLA's, internal clients, two people in an office -- you might have to worry about worst-case just a little, or potentially it might be at the core of your design).
With that missing, here are some general reasons why I would avoid all-in-one solutions:
Ridiculous analogy number one -- you can put a spoiler on a civic, and it might even do a little something, but that spoiler is really just a decoration. (If your security is all about hitting the right boxes for audits, sometimes you can "get away" with decorations. But this isn't real security, and we know it.)
Having a product that does too much can show up in product support as well. If you have a problem on your Everything Device, how much support is there for it? How long have they been supporting the 96th and 97th add-on capability of this device? If it's an afterthought, this could be trouble. How many developers does the vendor have involved in regression testing, bug fixes, and new code releases?
For hardware, often you can add modules that give you extra power in some areas. Cisco sells a lot of cards and modules that will offload crypto, or 'anti-X', or IDS/IPS work, and these go in everything from ASA's to 6500 core switches. Great idea? That depends. Can you afford another device, or will you do more with less?
And routers are still best at routing traffic, core switches are generally better not fooling with access-lists, firewalls are better off not running OSPF and BGP.
To end, here's another ridiculous analogy -- an athlete can be a fantastic swimmer, or basketball player, or gymnast, but that doesn't mean that the same athlete should also fill up a position on the football or rugby roster. If you're an IT person, and you have a set of requirements, you can have one device take up all those checkboxes on your list -- fill all the available positions on your roster. But if you've got the budget and resources to buy and deploy a stronger solution for each need you have, I would certainly go far that as a more sustainable strategy.