I have been developing a single page application for several months in SvelteKit, which emphasizes server side rendering and progressive enhancement. I spent a lot of time making things work without JS when it would have been easier not to.

Modals were the spot where I threw in the towel on perfect progressive enhancement. Like yes, I could make this work without JavaScript. But I'm not willing to take the likely weeks designing and building a robust solution for it which would definitely have to involve routing.

I like OP's post, because this is a decision I am wrestling with. Right now I only use modals for things like confirmation windows or showing a larger version of a photo when you click a thumbnail. Kind of like the use cases they outline. Ephemeral. But I am about to start implementing a payment flow that is more complex than just "enter your card number and hit enter", and as nice as I think it would be to have it in a modal, but having it as its own page/flow is so much easier in so many different ways, I think this tipped me over the edge.

What issues? It really seems like subjective experiences being pushed as best practice.

It's absolutely possible to build maintainable, accessible, linkable, fast modals.

I have no clue why someone would say those things are impossible or impossibly difficult to achieve.