[omra]

Diminishing returns: The amount of mine-able bitcoins decreases overtime, as well as the amount of reward decreasing as the hashing power of the network increases. Assuming the price of Bitcoins does not increase accordingly, the amount of money gained will go down.

Bitcoin Collapse: If Bitcoins collapse, you now have a bunch of worthless miners. Compare this to a quick sell it and done.

God forbid you gain a significant amount of network control, because you have a problem. Miners will probably begin to reject your transactions for the sanity of the network, because you could double-spend, and virtually create infinite bitcoins.

Also, how is setting up a business like this trivial? I imagine that a huge part of it (initial investment, buying power, and maintaining all these devices) could be problematic, although I could be wrong.