> You're stuck in Apple's app store which is equally as draconian as Chrome's and Firefox's but now you have to pay $99/year.
Safari extensions do have to be notarized (so $99/year), but notarized extensions don't have to be distributed via the Mac App Store.
> No IndexedDB.
Safari appears to support IndexedDB, with native support for IndexedDB 2.0 in Safari 10.1+. https://caniuse.com/#feat=indexeddb
> The extensions aren't usable on mobile.
Safari for iOS does support extensions, so I'd assume they'll use the same framework. https://www.idropnews.com/apps/11-best-safari-extensions-iph...