Watts are watts as long as the load is a simple resistor. As soon as you're driving a complex impedance with resistance, inductance, and capacitance it might be better to say that current is current. And many (most) solid state amps use current limiters to protect their output stages from excessive current draw by their loads. A SS amp that will happily say, 5 Amps into an 8 ohm resistor, will complain mightily if the phase angle of the load shifts towards 90 degrees and the impedance becomes much lower, drawing current the amp can't cover, so on come the the current limiters, at far less than the rated power.
Good SS amps have power supplies and output stages that can handle complex loads, but 'watts per dollar' amps have to cut corners, and the easiest places to do so is in the output stage current handling and power supply.
Good SS amps have power supplies and output stages that can handle complex loads, but 'watts per dollar' amps have to cut corners, and the easiest places to do so is in the output stage current handling and power supply.