I have a Sony Alpha a900, which was discontinued in 2011. I bought it, because it was the best full-frame DSLR they made, before switching to the SLT formfactor.

I really like like the speed-of-light latency of a fully optical viewfinder, and the ability to use the entire dynamic range of the human eye. I also installed a split prism focus screen, because it provides information about how far out of focus the target is, and in which direction.

I bought it in 2015, but since then technology has improved a lot, and image sensors have enough dynamic range and low-light sensitivity to be more useful than an optical viewfinder, it's possible that mirrorless cameras can be more usable than a stock SLRs, presuming that the sensor-to-viewfinder latency is countered by the lower shutter lag, compared to the time needed for a mirror to get out of the way.

Mirrorless cameras sensors have built-in phase-detection focus sensing, so they have access to the same kind of information a split image focus screen provides, spread throughout the sensor instead of just the center like most focus screens. Instead of just overlaying a box on areas that are in focus, there's no reason a mirrorless camera couldn't overlay information about the phase and amplitude of out-of-focus portions of the image, for example by drawing a line and tilting it to one side for when focused too close and the other when focused too far. As far as I know, no manufacturers do anything like this, so the split prism focus screen of an SLR viewfinder is still somewhat unbeatable.