The ROI is the most important.

I look at how long it takes for the machine to produce what it cost me. Now, if it's more than 9 months, I don't buy.

Several of my latest machines did not produce the BTC they cost me. Because of the low reliability of bitmain machines, the increase in difficulty, plus import taxes, it's hard.

If a machine that costs 0.1BTC only produces in it's working life 0.08BTC, there is no reason to buy it, it is better to keep the money.

I don't know how other miners do it, but for me in Europe it's no longer profitable, even with free electricity.