I fly missions at 300' over 2000 trees in a 17 acres area with heights up to 110' and usually get good results. To achieve this required:
1. 80% front and side overlaps
2. Overcast days
3. Low wind
4. Low enough speed to avoid any noticeable blurring in the photos.
Recently when I used the automatic exposure settings and defaults (speed up to 34 mph), I got blurry photos even though DroneDeploy reduced the speed to 12 mph. When I manually reduced the max speed setting to 9 mph, the photos showed very little blurriness. I notice the advanced settings page, which provides access to the speed control, is now changed and shows 10 mph default. So maybe the default could now be ok.
I review all the photos from a mission and remove any that are noticeably blurry. Almost all photos at turning points are blurry so I remove those. But you have to look more closely, at least at a few photos, to make sure there is not a small amount of blur in the direction of travel. If so re-fly the mission at a lower speed. Overcast days mean slower shutter speed so the mission time will go up. You need to find a balance between speed and mission time and photo blur. A multi-battery mission becomes necessary at some point. Or you could make separate, one-battery maps.
Bottom line: mapping trees can work well but you need to be more careful with the overlaps, altitude, flight speed, wind and illumination (with overcast being favored). Look at the RMSE results for your mission. My best is RMSE error is
4.5ft with X = 7.5ft Y = 1.3ft and Z = 1.4ft. To see these numbers click on the title of your Mission when on the Data page (this page says <- Dashboard at the top). If you get RMSE's in this range, I think your maps will look good. But as Gary points out, you need to fly high enough to achieve this.
I think part of the reason my RMSE is so low is that I fly only E-W dominate missions (1200' E-W with 70' N-S at the turn ends). This keeps the latitude (Y) constant for the long E-W legs with only the longitude (X) changing continuously. You could try flying some missions like this (or long N-S legs and short E-W turns) and see if it helps.