VCE(sat) means the load won't work at low voltages with a BJT. The required base current will cause an error term in the current sensing if sourced from the load's internal power supply, and if sourced from the DUT, this will increase VCE(sat) to that of a Darlington or worse, making the load even less useful. MOSFETs don't require gate current at DC, their only equivalent to VCE(sat) is (ID)(RDS(on)), and RDS(on) can be very low with a modern device.