I was stuck once by abusing `bind:` and other time with global exit transitions + sveltekit routing, but that's about it.
React on the other hand feels like a pot warming up and we're the frogs being boiled alive at this point. You can write some perfectly valid, but naive React code and it's going to have some footguns. We're so used to write idiomatic React code we don't even notice that anymore. You learn to use stable references in callback functions, to be careful using Context, and so on. But the list is not short, and it only grows.
Wanna write some naive code in svelte two days in? There's a good chance it's the same code you'd write a year later. Maybe that's what people mean by "just JS".
AFAIK WebAssembly can't do more harm than JS since it lives in the same sandbox, so that's not a concern? You're basically running Flash in a small VM.