First Monstrous humanoid is not a subtype of Humanoid.
Check the back of your bestiaries for creatures by type.
The emphasis on not changing creature type is based on the extent of the illusions power.
This is only a level 1 illusion spell the extent of the changes are quite limited to only minor things within reason. Obviously when compared to more powerful illusions the area and effects are greater. Within reason try to consider each level to be the cutoff of power for each. Ex. invisibility is an illusion + glamer, but it is higher level than disguise self so this would indicate that disguise self is not entirely capable of doing something as complete as full body invisibility.
A typical type of creature from another is so different that it would be too obviously different. Like a Human in disguise as a lizard man. The tail and bone structure are so different that you would look like a person glitching through the character model that is being overlapped on top of you. This spell is only an illusion with the Glamer type.
Glamer: A glamer spell changes a subject’s sensory
qualities, making it look, feel, taste, smell, or sound like
something else, or even seem to disappear.
In general if you go outside of the bounds of the illusion or are disrupting it in some way it either fails or can be seen through.