Is this card fast enough for 4K video?

Yes for typical 4K recording. The V30 and U3 ratings guarantee a minimum sustained write speed of 30MB/s, which covers most phone, action camera and drone 4K modes. Very high-bitrate or slow-motion 4K may need a faster card.

Will it work in a Nintendo Switch or Steam Deck?

It fits both, since they take microSDXC cards. Load times will be slower than on a faster UHS-I card with higher write speeds, but it works for storing games.

What is the difference between this and a UHS-II card?

UHS-II cards have a second row of pins and can reach much higher read and write speeds. This card is UHS-I, so it tops out around 100MB/s read and 30MB/s write and will not use UHS-II speeds even in a compatible slot.

Does it come with an SD adapter?

The listed box contents are the card and the original box. No SD adapter is mentioned, so check before buying if you need to use it in a full-size SD slot.

Is 128GB enough for a phone?

It adds a large amount of storage for photos, video and apps that support external storage. How far it goes depends on your file sizes; heavy 4K recording fills it faster than stills.