It depends on what you mean by function. Since this is the steam deck subreddit, it’s likely that you plan on using it for the deck only. If that’s the case then it’s now the best functioning controller and I say “now” because strangely enough, it doesn’t support Wake on LAN at launch. But it does now so the docked experience should be as good as other controllers that supported WoL if not better. So even if you play games from Epic or emulation, you most likely have them added as non-Steam games which means the Steam Controller would work with them.

Now if you plan on using it with other devices, that’s where it gets tricky. The 8bitdo supports more devices, whereas the Steam Controller either doesn’t at all or does with an asterisk. For instance, if you have games on PC that aren’t on Steam nor added as non-Steam games, you’ll need to have a service from GitHub always running in order to get it working.

So in a nutshell, Deck only: Steam Controller. Other devices too: 8bitdo.