The thing SD cards are (still) pretty bad at is random access.
That will affect some games more than others, games which are constantly streaming assets may see reduced performance (lag spikes), and will likely experience increased pop-in and such. But since the Steam Deck is not that powerful, you're not using incredibly high res textures anyway.
The games they tested load in all the assets ahead of time. Those are not really the concern, you can deal with slightly increased load times and the performance itself is not affected. A well designed game will have the level assets packed together so that they can be loaded sequentially. But for games (especially open world) that stream assets and allow you to freely explore, this is not really an option. They're much more dependant on sufficiently fast storage since streaming in assets from slow storage can lead to lag spikes and worse.
