What I dislike the most about the modern web and the unnecessary use of SPA technologies is how unreliable and error-prone it has made sites that really need to be simple and reliable. For example, banking websites, utility websites, government websites (like DMV, unemployment, and others), anything where failure or mistakes have a high cost.
These sites should stick to reliable, tried and true, pre-SPA technologies. But now that's "boring" and "not-modern" so they rebuild using React (or whatever is the popular SPA framework this month) and the result is often a huge degradation in site quality.
Because of the cargo culting, you see all sorts of Fortune 500 companies using this for fairly critical end-user tasks and failing at it. Air Canada did a redesign a while ago and I still can't book a flight on there. You look at the console and it's barfing all sorts of client-side errors and 500's. I would rather have a static, slow, reloading app that is reliable.
There are good uses for SPA's like ticket sites that have virtual queues and office applications. I just don't see the advantage SPA's give me for a lot of critical things I need to do in my life.
These sites should stick to reliable, tried and true, pre-SPA technologies. But now that's "boring" and "not-modern" so they rebuild using React (or whatever is the popular SPA framework this month) and the result is often a huge degradation in site quality.