Sony gives out a lot of "first ever" titles, and some of them don't always live up to expectations. But, the Sony a7R II is the first backlit full frame sensor with 42 megapixels--and it still earns a remarkably high image quality test score of 98/100. Sony is also still the only manufacturer offering a full frame mirrorless, and that high-scoring sensor is packed into a magnesium alloy body that's just a little over a pound. While mirrorless has traditionally lagged behind in terms of autofocus performance, testing shows that's not the case here, with the a7R II focus comparable to DSLRs even in low light scenarios. Most DSLRs also don't have image stabilization in the body, but in the lens. The a7R II has a five-axis image stabilization in the body (which means you can save a bit of cash since you don't need the lenses to be stabilized as well).
But fitting all that tech into a one pound camera body means there are a few sacrifices. Chief among them, the battery life. While DSLRs typically have over 1,000 shot batteries, the Sony a7RII, like the earlier model, is way behind here with a 290-340 shot battery life--you'll need more than one backup for all day shooting. The burst speed also isn't very impressive at just 5 fps. While that's similar to what other full frame DSLRs in this price range achieve, mirrorless cameras are typically faster because there's no mirror to move.
Since Sony is currently the only company with a full frame DSLR, there's not much to compare the a7R II to. With the older version, the sensor has been upgraded to a backlit design, which gives the image quality a bit of a boost. The a7 II (without the R) has half the megapixels, no 4K video and fewer autofocus points, but also comes in at about half the cost. The small size also means you may be paying a little more than similar full frame DSLRs, though it's certainly worth noting that the other DSLRs in the price range don't have the backlit sensor. If image quality and portability are top priority for you, the a7R II is a top contender, just know that means sacrificing battery life.