Vantaggi
There are some very talented people working at Hopper - it varies by team so your mileage may vary. The company has unlimited PTO. You get to learn how the travel industry works, which maybe is interesting to you. Hopper has committed to a relatively modern micro-service architecture with a Scala backend, so lots to learn there.
Svantaggi
Hopper is into STO (single threaded ownership) in a big way (they lifted this, as well as many "core values" from Amazon). This means that each team is theoretically responsible for a one or more product features which typically translates to some components in the mobile apps (iOS/Android) and one or more micro-services (of which there are dozens). In practice, since there is one code base for iOS and one for Android, you have dozens of separate teams all contributing code with little way of enforcing consistency in the UI or in the code itself. Unit testing is inadequate and consider yourself lucky if you find any actual comments in the code. High level executives have told people that "your opinion doesn't matter - it's only what the customer wants". This is an eyebrow raising concept which reminds one of the old automaker quote "If I had asked people what they wanted, they would have said faster horses." Hopper instead makes all decisions based on A/B tests. This is fine for deciding on some aspects of the application, but it doesn't take an A/B test to understand that not all customers want to "tip" Hopper by default when booking an airline flight (quite the dark pattern) or that the hotel pricing display is confusing. There is no central engineering function - everything reports up to the STO leader which means in practice engineering reports to product. This means there is constant pressure to make short-sighted decisions to deliver the next product feature that inevitably mean taking short cuts and piling on more technical debt. At some point a reckoning will come and I'd be cautious about being there when it does. It is especially bad in some teams where the STO leader doesn't have an engineering background and doesn't value unit testing, solid code review practices or general architecture. Regardless of your seniority or experience, you are expected to just pull tickets off the Jira backlog like some sort of short-order cook. Heaven forbid you try and take the long view and advocate for putting in place better architecture, testing frameworks or tools to support your job.