Sony A7R disassembled
Cameras without a mirror are not only more compact and lighter than an SLR, the chance is great that they are also (in the long run) more robust and reliable than an SLR, because they contain fewer parts, and because there are fewer units of the camera that have to be precisely attached to each other with screws - with the risk that such screws have to be adjusted again after a while. For example, this can cause front-focus or back-focus, when the AF unit of an SLR camera is no longer perfectly aligned with the sensor.
On LensRentals, there's a photo series from a Sony A7R, a mirrorless camera with a full-format sensor, which has been taken apart. What's impressive is how well designed the Sony A7R is. The camera consists of few parts and there are indeed few parts that have to be calibrated in order for the camera to work properly. That doesn't only make the production cost lower, but the chance that your camera breaks is lower and any repair costs will be lower as well. An experienced repair person needs less time to repair a Sony A7R than an SLR camera... if it does go to pieces.