So here’s the issue — you’re not using web components, you’re using polymer. And polymer is a third party external library, no part of which is built into the browser. It’s much heavier and slower than preact, and has a much less familiar API to developers. Try using web components without polymer and see how that experience goes — spoiler alert: it’s not pretty.
Basically, the big difference here is virtual dom diffing and rendering. This feature is essential for a modern web app, and web components not only don’t have this feature, but there aren’t any plans at all to add it. As such, you must use a third party library in order to get it (polymer, skatejs, etc), and at that point you have lost the primary benefit of web components — that they are browser-native and a web standard.
You speak a lot about build tools here and how they aren’t necessary with polymer. This is also rather confusing. Polymer requires build tools for production optimization and efficient local dev, just like any other framework. And react etc. don’t actually require build tools at all — it’s only if you want to use jsx that it requires this. There are plenty of other ways to build out vdom trees by requiring templates at runtime (like polymer does), or writing it in hyperscript instead.
It could be argued that if you have fast internet and a good mobile device, the performance impact is minimal, and that most people use google so it’s ok for SEO, and that you don’t really care if people share your site on social media and it turns out blank, or with the same meta for every view, and that you could not use build tools and not optimize for production to make things simpler. But not only are all of these extremely weak arguments, but the real question is why make these sacrifices?! You are not using web standards or reducing your lock-in to frameworks by using web components, because you’re using polymer, which is a third party framework. You are giving up a TON of performance benefits and adding a number of extra infrastructure steps (polymer cli, prerender, carefully calculated selective polyfills) for literally no reason. You can just use react/preact/vue or any other better third party framework, and get a much more supportive community and syntax, easier hiring, simple, well-tested SSR, no and need for polyfills at all, with no downside.
I want web components to be good, don’t get me wrong. I experimented heavily with them to see if there was a chance that they could be used on their own without third party framework support to build an app, because this would be fantastic. But unfortunately right now, they are not anywhere close to ready, and it’s not worth using them. And polymer is just flat out worse than your other options, since it consistently needs polyfills, uses foreign conventions that developers have not adopted despite its years of existence and marketing by google, and it doesn’t have server rendering at all.