My experience with a P4P drone is different. For missions with 80% overlap and nadir-only pictures taken on an overcast day with low wind, the vertical accuracy is much better than the horizontal accuracy as reported in the RSME values for the mission. X variation is 10' while the Z variation is reported as 1'. This should be expected since the P4P uses its Barometer and IMU to determine the relative and absolute altitude recorded in the Exif data of the still photos it takes. The relative altitude accuracy of a barometer can be suburb during a 10-20 min typical mission. These good altitude values (much better than the GPS x,y) are then used by DroneDeploy to create the 3D model and set the elevation reported by a Location click on the map. When I draw a measurement line across a 100' length of driveway with a slope of 1/4"/foot, this slope shows in the profile plot with noise of less than 0.15" (see below). So my experience is that the relative elevation accuracy for textured hard surfaces (exposed aggregate finish on concrete in this case) over distances of probably more than 100' can be quite good!
On the other hand, the absolute elevation accuracy without GCPs or using the Auto-calibration Technique (ACT) I described in Steep Terrain Question, is absolute garbage. I regularly see 500' differences in elevation from missions flown on different days when the barometric pressure is 0.5" different. But I use ACT to remove all of this variation at zero added cost. The DroneDeploy report elevation for the 2D map above was 768' before I corrected it with ACT.
Your experience may differ depending on the drone used and conditions during the mission. Again, my best results are from missions on overcast days and low winds as per DroneDeploy's top-10 hints for best results.
I find the vertical accuracy of the combination of a P4P with DroneDeploy to be way beyond my expectations! Oh, and by the way, the above results are for missions flown at 300' above take off point. So the 0.15" elevation noise for photos taken at 300' is stunning!