Prologue as Spoiler
A prologue need not be the most interesting scene in a book. Sometimes the prologue is used to tell the reader where the story is going. One prologue, transplanted from the middle of the book, was simple news of a character's death.
Whatever emotional impact there would have been from learning that in the middle of the book was lost. I suspect it would have made me sad.
The author had a problem. The first half of the book was about the main character trying to live her life, and the second half was trying to discover how the dead character died. That's difficult to handle. The author was worried that the first half of the book wouldn't be interesting enough by itself. (Personal Communication).
I thought the first half of the book was interesting by itself. As a part of this, but only a small part, that included tension and suspense as I worried about why a character was missing and when/if she would return. The prologue would have spoiled that (if I had read it).
Also, a reader who knows a character will die will not become attached to that character. I didn't know any better, so I became attached to this somewhat unlikable character. So her disappearance meant more to me.
But, any event from the middle of the book is a spoiler. And it's probably worse for an event selected to tell the reader where the book is going. In the above book, the reader would know from the start that she is going to become really angry at him. So the conflict and tension on that issue is gone. It's corrosive – we cannot be inside her mind the first half of the book naively hoping things will just get better.
The extent of the spoiling can be reduced by making the prologue less meaningful. If you ever read a prologue and have no idea what is happening, perhaps because it's just some intimations that something exciting might be happening, that technique is being used. But it's the standard trade-off – the more exciting the prologue, the more it tends to spoil.