I haven't done an install, but I have seen one done. The person knew what they were doing so it looked relatively-straightforward.
Replacing the flush valve seal is actually pretty-straightforward: you remove the actuator plate and the actuator, reach in, remove the thingy that is holding the valve in place, and remove the valve by bending it forward and sliding it out. You replace the rubber doughnut and put it back the way it came out. Same thing for the fill valve if you ever need to fix it or to adjust the water level.
They have at least tried to make it easy, and folks routinely put these things behind nice, finished tile walls, confident that they can replace anything that matters by getting to it through the opening by the actuator plate. If the tank cracks, that's another story, but apparently that is very rare.