I agree wholeheartedly that frontend work is very, very complex (and has only gotten more so in recent years), but I disagree it's the hardest part of the stack

I agree wholeheartedly that frontend work is very, very complex (and has only gotten more so in recent years), but I disagree it's the hardest part of the stack simply because you typically don't have to worry about scale. Imagine a system processing billions of records per day/week/month, dealing with data caching, data warehousing, providing realtime notifications to users and external systems, managing queue workloads, handling inbound requests from various APIs, syncing data between backend systems, running multiple data stores and scaling across multiple regions, redundancy, and handling incidents that arise on the backend. This is typical for complex web software. A frontend system that provides the interface to that backend system will usually not have issues arise in the middle of the night due to the automated processes happening on the backend, will not have to worry about how many users are using the system nor how much underlying data there is. No matter how difficult it is to do the initial development of the frontend (or to keep that code updated when browsers change), at the end of the day once the development is done it's static code interfacing with a much more complex underlying system that requires constant attention as underlying data and business requirements evolve.