This is not by any means a fact. I am not a expert on the cryptocurrency field but this is something that anyone will consider before jumping into mining.

  I recently checked genesis mining and currently is considering purchasing their 30MH/s ethereum mining. Everything seems legit but I had 1 major problem. Since ethereum is getting big quickly that means more people will eventually start mining ethereum, which increases competition. In this case, competition is referred to as difficulty. Overtime if more people start mining, its difficulty will rise. 

Bitcoin uses TH/S to mine, which means it it mines 1000000000  (1 trillion) hashes per second whereas currently Ethereum uses MH/s which is 1000000 (1 million) hashes per second. Overtime, when ethereum becomes as big as Bitcoin, 30 MH/s will mean nothing in the long run.
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