Downside is that clients have to know a about every type and every future type for it to render properly. Think of a world where there are hundreds of different app-specific kinds. Fallbacks are super important if we are going to start doing nip27 embeds everywhere.
Discussion
Agree, fiatjaf and others are writing a NIP for fallbacks. There is also some discussion around user defined kind handlers, clients could use this to open an unsupported event kind in a user defined app.
NIP-31 or 32 (iirc) alt tags. Human-readable context for any event kind.
NIP-89: unknown-kind app handlers+recommendations.
These two nips is all we need to have transparent Inter-kind Inter operation and compete with super apps’ UX without the centralization