I have just agreed to buy a Mjolnir 2 after a lot of research, including this thread, and want to test my understanding. It think its of general interest because there is so much less online discussion about the Mjolnir 2 vs the other headphone amps, but it is an interesting part of the Schiit lineup and story.
As described here, Mjolnir 2 was a radical redesign of Mjolnir. The biggest change was the switch from solid state design to tube design. Schiit decided to do something funky at the same time and launched the LIIST solid-state 'tubes' which work in the Mjolnir 2 and other Schiit tube amps. Schiit marketed the bundle as providing the best of both worlds.
A few years down the line, Mjolnir 2 is still in the lineup as the end-game headphone amp. LIIST seem to be reviewed as decent, but not vastly better than tubes and were ultimately discontinued because of the complexity of manufacturing. For someone new coming in it can be condfusing to see reviews of the Mjolnir 2 that are based around it's capabilities as a solid-state amp, and which don't place it with the lineup of other Schiit headphone amps and modular amps. It seems Mjolnir 2 used to be differentiated as the balanced amp with tons of power, but it now has to share the stage with Magnius. It seems that Mjolnir 2 used to stand way ahead as the flagship tube amp, but it now has to share the stage with the Lyr 3 and its newer design including Continuity.
In a recent online discussion, it seemed like Jason was suggesting the sheer size and heat output of Mjolnir were also hurdles to its popularity.
Many people who own the Mjolnir 2 love it. I hope that's my experience too.