I'm glad you aren't taking the barrage of "get better hardware" as an insult. 99.9% of people here want a very solid, very reliable, system. So the feedback you are getting is because you're opting out of a solid/reliable system. :P You're choice to use the hardware you have goes contrary to that, so several people (including myself) are questioning if FreeNAS is a good choice for your server.
I had an ARC-1280ML-24. Few tips.
1. SMART will NOT work properly on Areca controllers. Tried to get it to work and the bottom line is you really can't. Since that card isn't even recommended by the developers and is so rarely used there was no point in spending developer resources to hack something together to make it work.
2. You're already in that territory where everyone is telling you to replace it for your own data's safety. I hacked together a working solution as a script and emailed me nightly, and I later found out that even that didn't work as well as I had hoped and I had 2 bad disks in my RAIDZ2 zpool for I-don't-know-how-long but it could have ended badly for me. After rebuilding the failed disks I realized that trying to make it work in the interest of saving money, blah blah blah was NOT worth it.
2. Areca stuff is basically "supported" as in "the driver exists" but you are very, very much in the minority. I would never trust it with my data (I did, and I learned that lesson).
3. Do not be surprised if suddenly you find yourself with a zpool that is unmountable and cannot be accessed. We warned you. :P
In short, you are circumventing primary mechanisms for verifying the health of the system and disks in FreeNAS. Buyer Beware! If this is okay and you are okay with the risk, feel free to continue on with what you are doing. Just don't be surprised if you have serious problems, ask for help in the forums, and the answers are "cannot help you.. the hardware corrupted your data". Even calls to iXsystems for repairing stuff what we're trying to explain won't get your data back. :(
I wish you luck in whatever choice you make. :)