Three thoughts:
First, the Grant function model is inaccurate. An IJ can only lead with Ji, so it is JiPi. Si is the auxiliary of the ISTJ. Ni is the auxiliary of the INTJ. Both share the common dominant function, which is Ti.
Second, the worst thing is perhaps not being seen as evil or cold-hearted. The worst trait is having no trait. Just mediocre, neglectable, hardly noticeable. In today's society, one can leave an impression for being tough, cold, ruthless, etc., and yet still be remembered. Negative traits at least indicate characteristics. A 'nice guy' however, does not have anything special. They tend to be overlooked and their existence is like thin air. To say somebody 'nice' implies that 1) people don't know much about this person, and/or 2) this person has no noticeable traits. To say "you are nice" simply is yet another way of saying, I don't know much about you, with some politeness.
Third, the Jungian type ISTJ is an empirical and critical thinker, not the average nice guy. The ISTJ being a nice, average Joe is a misconception. Ti implies critical and independent thinking, and the TiSi combination suggests empiricism.
ISTJ - TiSi, empirical critical thinker. INTJ - TiNi, creative critical thinker.
For example, Kant was clearly an ISTJ, TiSi. There was hardly anything intuitive about him. He was strictly logical and rational, and was quite grounded. You can even argue that he had OCPD (a PD common to people with High Conscientiousness). I wonder why people type him into INTP and keep saying he was in a 'loop'. A loop does not exist. In many cases when people have difficulty typing someone, it is because the Grant model does not fit. Using the Jungian model, it becomes immediately clear.