The comparison is faulty because AM is a company and the other alternatives are hardware.
Why is that relevant, you ask? Well, AM will (hopefully) continuously renew itself and its strategy in order to keep earnings high, whereas the hardware simply gets outdated and earns progressively less and less as time goes along. So: buying hardware may earn you more in the short term, shares in AM may earn you more in the long term.
Comparison is NOT faulty. It is a comparison of "investment" (whether in hardware or a company) and "return" (whether mined bitcoins or dividends paid out as bitcoins). I think most people will agree that the value of hardware (GPU or ASICs) will generally depreciate over time, so that is a "known." The big unknown is AM, which as you point out is a company. We hope they will continuously renew itself, but anything can happen.