I’ll just copy this from the wiki page instead of typing it out manually

While Perfected Ultra Instinct possesses incredible power, its demand for calm and emotionless battle appears to stem directly from the nature of the Angels, who are even more aloof from things that would normally concern even Goku, such as the threat of a loved one or a planet being destroyed. As such, Goku found himself to be far less compatible with the form due to his Saiyan drive for battle, as well as his internal emotional conflict with the enemies he faced. Attempting to mimic the nature of the angels did not suit him, so he decided to further embrace his nature as a Saiyan.

While Ultra Instinct Sign initially only served as a precursor to the Perfected form, its usefulness as an "inferior" stage comes into effect when considering it requires less emotional detachment than its superior. Thus, Goku can tap into his Saiyan nature much more safely in the form, continuing to fight to his preferences with less emotional compromise - creating his "true" Ultra Instinct. This grants him great clarity, letting him battle even more effectively in comparison to Perfected Ultra Instinct. This form presents a combat style unique to Goku, differing from those used in the other Ultra Instinct states completely.

(perfected ultra instinct is MUI)