One huge drawback of AppCache is that it’s caching exactly one block of data. This must obviously contain the Discourse “App”, and shouldn’t contain all posts (of course), so the actual data (the posts to be cached) has to be put into something like localStorage, which seems to be often limited to 5 MB.
(I have some experience with AppCache as I have built one webapp with it. AppCache is unflexible, confusing and punishes you for any mistake you make, but works pretty well if used correctly.)