Either paragraph is only valid for Scala < 2.12

[loicdescotte]

The book says that it does not work in for comprehension without .right projections, and that Either can lead to mistake if you don't remember that left is for errors and right for successes, by convention.

It's no longer true as Either is right biased since Scala 2.12.

[loicdescotte]

Personal opinion : Either is now very convenient to use, and different from Try as it can have custom error types (ADT for instance) for errors, not only exceptions - for example it's nice for functional (non technical) errors.