The most glaring flaws I found

[meowthblep]

1. No Saving or Save States: You cannot save your game and go back to where you left off, not even via save states. Other emulators have the easily implementable workaround of downloading the save states and plugging them back in when you come back. Another workaround is saving the save file/state as a cookie.
2. No controller support: Some games are better played with a controller, however, there isn’t support for this, so you have to use AntiMicroX or another tool.
3. Some emulators don’t work well: MAME console emulation isn’t the best for playing certain games, and some games straight up don’t work. I recommend adding different emulators for games like PS1, and adding new emulators for N64 and DS.

[jbaicoianu]

1. We definitely implement filesystem persistence via BrowserFS, although it's not straightforward to configure. I thought we had some examples which cover this, but it seems that most of the examples are indeed showing read-only setups. @db48x do we have any examples which show how to set up a UnionFS, or is that something we only did for the Archive.org implementation?
2. Gamepads are supported via the HTML5 Gamepad API, so as long as your controller is visible to the browser, it should be available to the emulators as well. You may have to ensure that a button is pressed on the gamepad before the emulator starts, as some emulators scan for controllers at start, and won't pick up controllers which are activated later.
3. MAME definitely has some platforms it doesn't handle as well as special-purpose emulators. If those emulators have WASM builds, it should be possible to use them with Emularity as well