anything where you need popups or tiled windows, code editing, rich text features more complex than "render markdown into a div", heavy content like videos, multiplayer, real time chat, anything that has to work offline... htmx is only good enough for something like a company homepage or simple shop not big complicated apps. its actually the same reason i dont like htmx, the whole "true REST" approach is about making everything depend more on the server with a thin client that can only do a couple very simple things with the loaded page. if your connection to that server is slow or unreliable your whole app breaks.

its also the perfect opposite of "true web3" and ethereums original vision, where you load all static assets from ipfs, most app logic is client side and the server or blockchain only comes in (json api, no html fragments or full pages) when you need to interact with other users. still believe in it even after the crypto bros took the name for a bunch of scams and hosted everything on cloudflare anyway. the only thing they have in common i could find is no bundling but for different reasons - everything on server vs compiled libraries shared between apps.

Facebook, Spotify, Figma, really anything interactive beyond sending a simple form.