Disks breaking down is not the main risk. From own experience (now have 80+TB) i would estimate at least 80% of all burst drives, used or new, will last for more than 2 years. The future price of burst is the main risk, as i see it. If burst falls, you will need more time to break even, and if burst rises, you could make a lot more than your initial investment. Cost of watt is an issue also, but a lot less of an issue than for GPU and ASIC mining.

I decided to keep all the burst i made, which makes me much more sensitive to burst price swings - as long as i see a bright future for burst, i'll keep my coins.