I played a 2 day tournament with 20+ ppl and came third a few weeks ago. STD ended up in 2nd place as well, with Tzeentch (changehost) taking first place. I ran Undivided with 0 marauders, 15 warriors, 3x5 knights and 5 marauders horsemen in a ruinbringer and some other stuff in ravagers.
I don't think you can play STD like a conventional army e.g. you take a hammer that deletes whatever is in front of it and then start trading units with the opponent. Instead it is about control and not letting the opponent play his game. Since I like the challenge of not having a top tier army and prefer having a few tricks up my sleeve rather than a straight forward combat army STD has proven to be perfect for me.
The reason I went for Undivided is the auto immune to battle shock which makes units invaluable roadblocks. It also frees up a few CP which are extremely valuable in STD. Furthermore, the combination of the generals 6++ with the warshrine 6++ and rr saves from 2 x orracular visions and 10+ warriors makes the army incredibly tough. However, for future tournaments I want to try to run half Khorne, half Undivided so I get the best of both worlds. This forces me to bring heroes of both marks and gives me a choice of being more offensive or defensive in ravagers.
Regarding the low dmg from our units I agree, it is too low to whipe any unit off of the table with certainty. But 5 fully buffed knights does enough dmg to cripple most units so they do not pose as much of a threat any more. As for the block of 40 mortek guard, don't charge them head on. Tag them with one model and render them useless. Maybe STD have to be played like onions /Shrek (not Ogors), with several layers that you let the opponent peel off one by one?
Lastly, for me it is much more rewarding to end up in 3rd place with a 2nd tier army such as STD than winning the second largest tournament here in Sweden as I did with Skaventide last spring. It was just not fun to steamroll whatever unit was in front of my monks.