@tyil dunno about ffmpeg.
But if you had a codebase that uses e.g. Redis and an AWS object store, in the WASI situation these are replaced by a keyvalue and blobstore standardized interface. And when your wasm component moves from the cloud to your local pc the implementation of these interface may dynamically switch to an in-memory KV store and the filesystem.
And your builds need not worry about all the different processor architectures.
Performance is near-native. Wasm is low-level stuff.