Mere hours after the release of Nintendo Switch Online, a group of players managed to hack it and manually add different NES games. For example in the video above, a player loaded Battletoads while apparently clicking on Mario Bros.
As it turns out, Nintendo just used basic .nes files and once a player saw what it was, they seemed to be appalled. The player in question goes by the name kapu or @KappucinoHeck on Twitter and they found out that emulating custom ROMs would be incredibly easy, provided any player had basic guidance.
They didn't manage to figure out how to change the games' cover art though, in order to "make it fully legitimate" but found out that players could even play same games online together provided they had the same setup. KappucinoHeck doesn't really know at this time if Nintendo is running some anti cheat software that would throw a wrench in modders' gears though.
Furthermore, Polygon asked kapu how easy it was to add custom ROMs, at which point they explained that the biggest obstacle would be for one person to get experience with modding Nintendo Switch content in the first place, which is not that hard with proper instructions. After that, the process of swapping out ROMs would be "insanely easy".
Kapu's working theory is that the ease of access to ROM swapping shouldn't lead to any potential security risks or vulnerabilities, but it was rather a side effect of Nintendo making it easier for themselves to add more games in the future.
It remains to be seen if Nintendo will wall off any users who uploaded custom ROMs to their Switches but considering how easy it seems to be, they were practically asking for it. On the other hand, since they did all in their power to shut down any sites hosting NES ROMs, it wouldn't be too hard to imagine the company dropping the ban hammers on users for having that kind of fun on Switch.