I did once cut some steel. Okay maybe not quite. I ground a flat spot on a super hard motor shaft. My file rubbed off of it indicating that it had to be really heat treated hard steel. I did it by clamping the motor and then manually jogging the bit slowly but surely. The result was what I wanted, it did carve out a perfect flat spot.
I think the mpcnc can cut steel…but Ryan is right it will take some serious coolant and certain conditions. With really light trochoidal passes I think it can but not at a speed that would be worthwhile.