I think the pairing works well.
As to the comparison with the Schiit stack, the Oppo offers a lot more functionality and supports both single-ended and balanced operation. It operates purely in class-A in the headphone gain path. The headphone gain circuit is really a gem. All of the additional functionality, flexibility, and sprinkles on top are nice but not really in the same league. On the upside, they all come in one box, play nice together, and don't need any additional cables or thought.
Mjolnir has a lot more power, but is balanced only. It delivers more power in class-A but can also transition into A/B mode to go well beyond that. With that said, I would be surprised if the amp transitioned into AB while driving the LCD cans.I haven't heard Gungnir (I've had experience with the Uber USB Bifrost), but I would have a difficult time imagining the Oppo's DAC is superior.
So I think the real decision here is do you want to spend $1200 to get a lot of flexibility/capability/bells and whistles and a nice all-in-one package or do you want to spend $1500 and get a more single-minded system that is arguably better at each of its dedicated purposes?
Both companies have excellent service and stand by their products. Both offer returns, though Schiit's policy sounds more like one that facilitates a trial whereas the Oppo one sounds more like "if it doesn't work to your specifications."