Why? Because I'm a second class citizen in a java IDE. No thanks. The marketing is also quite clear on the target audience https://www.jetbrains.com/idea/features/

Not really, tbh. IntelliJ Ultimate + the Ruby extension is RubyMine with a different name. Likewise with Go and JS, and I'm sure it's the same for php etc. but I can't speak for those. I've used this setup when I was testing out a bunch of languages for fun.

The plugins are not updated as quickly as the language-specific IDEs, sometimes they are several months behind on newer features. Not really a big deal if you wait a few months to update your Python version, but it's annoying if you update early.

I used to use only IntelliJ until a few years ago a Python feature I wanted was in PyCharm but not IntelliJ. I think it was related to the typing module? I can't quite remember. I've just stuck with PyCharm for python ever since.

edit: But also, I've never had any issue copying my key configs between PyCharm, Rider, IntelliJ.

But they could solve that without rewriting the whole platform

It is not true for C#, you need Rider for that. That is the only exception I am aware of, though.

It is my experience as well. I only use CLion for Rust and C. All the rest (Python, TS, Kotlin, Ruby, Scala) I do in IntelliJ.

Be that as it may, I've used Webstorm and Intellij Ultimate with JS plugins and the experience was pretty much identical.