What I understand from the STS, is that the system prevents you of being in a severe out-of-trim situation, helping to keep a good level of elevator authority in all situations. At least at my previous airline, where I flew the NG 700/800, a old instructor told me that on every takeoff we were taking off with plus 1 unit of forward trim, and that was to prevent tail strike, he said. He even showed me a page on the DFDAU showing the actual position of every flight control surface, and he proved me that the actual trim position was always exactly 1.0 unit forward compared to the wheel trim that we usually use to set the T/O trim.
Since we were taking off with a more forward trim setting, that caused a tendency of the airplane to accelerate after takeoff, and so all pilots were over accelerating, most of the time unable to keep the liftoff speed, simply because the aircraft was slightly out-of-trim. Then STS kick in, moving the trim to the up position. Then the acceleration altitude comes, and you don’t understand how the **** the STS is trimming the plane up, but the F/D is calling you to pitch down and you as well are commanding elevator nose down.
Bull**** or not, that was what I heard about it. Anyway, everytime I saw the STS working, it was very smooth, making just small inputs, easy to identify. I can’t see how it could be dangerous in anyway.