The V20, G6 and V30 DAC is designed for full size cans. It has nearly 2Vrms output when it senses >50 ohms on the output. It may not be as good as a high end desktop rig, but it has enough output to reach reasonable levels on inefficent headphones that other phones cannot reach.
Can it beat a good desktop rig? No, but that won't fit in your pocket.
Agree. My G6 can easily drive my Z1R and DT1990. It's not endgame territory but it does a surprisingly good job.