That would be one risk. Another would be variance in the time between blocks that you solve. The fact that the "average" with 60 GH/s is currently 1 block every 16 days doesn't guarantee that you'll actually get a block that regularly.  You could be zero blocks for 48 days by which time the difficulty could have already increased on you 4 times (wouldn't that be unlucky?).  You could also be 3 blocks in 21 days and have the difficult only have increased twice by then (now that would be lucky).

If you are pooled mining, none of that matters as much, since you'll get paid for the work your miner does regardless of whether it ever solves a block.  Unfortunately, by reducing the risk of being unlucky and coming out behind the average, pooled mining also reduces the potential profits of being lucky and coming out ahead of the average as well.