It's not GM that is the "scam", its n000bs that don't understand the business of mining. As the mining difficulty rises from n00bs and their mothers getting onboard and/or the price of the coin corrects from a mini-bubble, everyone earns less and less until the point where earnings drop below the GM maintenance fee (if any) and hence, no further payouts. This isn't an investment done for you, it's a business and you have to treat it as such. If you can't crunch the numbers properly before going in, stay out so you don't get burned.
95% of the time, buying the underlying coin is far more profitable than mining. There's only a few situations where mining is superior to buying but that's not the business that GM is in.