The mining pool is 20%, not 15%
The profitability of the miner is far better than most any tribe right now. Last I checked ENG Miners, for example, took years to break even.
You can also unstake them and sell them at some future date.
STEM also has far fewer tokens than any other tribe (about 50% of what Leo has for example).
RE: STEMM, Bad Miner, Good Purchase?