Swapping cash with BTC is not easy. If you want to buy sizable amounts of bitcoins, you need to go to an exchange. Exchanges ask you to complete KYC formalities. Now, the IRS will have your entire history.
Pretty much this. The "swap cash for BTC" would be the weak link in this money laundering scheme, but I guess it could be doable if you go around exchanges and use P2P exchanges on Localbitcoins or something similar. Also, if you make a lot of BTC with these mining rigs, then it will probably raise red flags when you exchange the BTC for cash through legitimate exchanges (or even P2P exchanges if done in a high enough quantity and frequency).
So, in the end I think you're better off simply keeping the cash in your mattress and using it for small purchases that won't bring attention to yourself (no mansions, yachts, sports cars, etc..), or using other means to launder the money.