Is it a risky investment? Yes
Because of the warranty? Not so much
I think buying a CoinTerra box is risky on a lot of levels, but not from the perspective of a short warranty. I have been a computer enthusiast for many years, and have quite a bit of experience with HW failures as an active member within the OCing community. The 28nm node (the manufacturing process which CT is using) is not bleeding edge technology. I believe they chose it because it is quite mature at this point, and the yield should be quite high (very few chips will be DOA). From my experience, if a chip is going to fail, it will either do it almost immediately (within a week or so), or not until its been run hard for a few years.
On top of that, even if one of the chips were to fail outside the warranty period, you'll still have 75% of the capacity you paid for, and the chance of loosing more than one of the chips is exponentially greater.
I bought a CT box (I'm also just a home enthusiast), and researched many of the risks involved, but the warranty is one I'm not too concerned about. Good luck
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