Memory management is pain. If you want to optimize, you're forced to 4k pages. Runtimes are fragmented. Wasi is a mess. Async and threading is awkward. Whenever you need to integrate with hardware you need platform specific adapters.

In practice, whenever you need more than a singlethreaded app with http/serial port, the "run everywhere" breaks.

Don't get me wrong I love WASM but we're not there yet