Manipulation vs Inspiration
Ok, indeed, it's overrated and as Dominic Monkhouse points out, everyone who has not been living under a rock must have already heard about this book! The book is overall so repetitive and verbose that at some point you need an incentive (like a chocolate bar or something) for yourself to finish each chapter after you have finished the first few chapters.
BUT, still, I think most negative critiques around this book (or in general, the concept of starting with WHY) originate in the very reason this book is written, and that is, don't be on a bandwagon, be inspirational to your customers/colleagues, challenge the status-quo and be transparent about it.
Critiques mention the importance of starting with WHO in the modern days to focus on Core Customer as an alternative to start with WHY. Although "who" is an important factor, the main purpose of the book is exactly to contradict the idea of starting with who! It is to beware leaders of companies, of focusing too much on revenue-driven metrics and customer manipulation which is a strong factor in sales and marketing and instead, to draw their attention to the main purpose of their companies' existence.
It also does not mean that with a clear vision and purpose, success will follow and customers will respect your decency. But without an inspiring cause, the rest is only about manipulation and optimisation to milk target customers dry. You might still generate revenue for as long as your wily mind finds ways to customer attraction and engagement, but you cannot expect your business to be a sustainable one. Or as Simon Sinek states, "manipulation leads to transaction, not loyalty".
Overall, the book does not provide you with an easy-to-use sexy-acronym framework, 7-blah-blah model or technique that you can directly apply to your business and be happy about your progressive business intelligence. Rather it lets you think and incorporate a critical mindset to always question before act. Something obvious for all critical thinkers but almost always neglected when people are overloaded.