Indeed, there are no difference between good compatibility and amazing compatibility and no difference between bad compatibility and awful compatibility. In addition, puzzle pieces always being in the same color does sometime make me think if the compatibility is good or bad when quickly looking over.
It is restricted to Growing Together, as it is heavily tied to Sim Characteristic likes and dislikes, which is tied to the pack. Without it, compatibility between neat and slob sim won't be affected at all.
Can't help but bring attention to the post where I looked into it.However, Sim Characteristics also takes into account cross-pack traits (except Horse Ranch).
What I do in my game is choose around 10 Sim Characteristic likes and dislikes for my sims (usually 5 likes and 5 dislikes). This however causes the effect where so many compatibilities are in the extreme (amazing or awful), and as I have a lot of pre-made sims, it will be amplified even further as the final score is the sum of both sim's scores to each other. So yeah, no differing compatibilities like it was with The Sims 2's chemistry system. NPC sims usually randomly get 1 Sim Characteristic preference, which may be either a like or a dislike.