There is a difference. When someone is disillusioned, they have the feeling that their previous perspective was an illusion, and now they feel they perceive the disheartening truth. Disappointment is generally a broader, probably less significant, perhaps more temporary upset. I was disappointed that the book I was looking forward to didn’t cover a topic I hoped it would. But I was disillusioned when I read chapter 5 and realized the author I’d thought I liked, is actually someone I can’t respect.