You'll be able to inspect the oring at reading glasses magnification and see if they are degrading. They won't be smooth. They'll be pitted if they've been breaking down due to a reaction for long enough.
You're right. It's the same type of reaction as with hazing, cracking plastic tanks.
The multi-pack o-rings that some get, often online for cheap, or at automotive stores, big box stores, etc., are made for different applications, and just because the size is right, doesn't mean the material is right.
There are several cinnamon flavorings used in ejuices. Some will have an effect similar to dry rot on o-rings that were designed for a water faucet or in an automotive application.
As mentioned above, it was more than just cinnamon that was giving those tanks a flogging...
I'm no materials expert though. I just stick with Buna N from a credible parts outlet.
I have no idea what the fast tech o-rings are made out of beyond "Silicone." A quick google search shows that authentic and non-authentic both just say "silicone" or nothing at all. I'm going to try and use just the spare o-rings that came with tanks first and the Fast Tech ones as a least resort but I'm glad we talked about this, I'll be on the lookout.