Inside publishing offices, people who give a lot of blurbs for books are known as "quote whores." It's very much a game of who you know, who your editor knows, who your agent knows, etc. It's not to say a blurb is worthless, but a blurb from someone who doesn't give a lot of them may be worth more.
- Expertise match: e.g. Elon Musk endorsing a rocket engines book vs history book
- Strength of endorsement: e.g. "This is the best book I've ever read," vs "I liked it."