Works well for me. You just type with one hand without ever lifting your thumb. You do need to set the correct language, and if you mix languages, your phone's OS has to be able to deal with that, but at least on iOS that is the case and the prediction is very good. If the first choice isn't correct, usually the first or second displayed alternative is what I wanted.
There are two caveats, though. Firstly, I type differently when using swipe. As it favors words from the dictionary, my texts sound less spoken and more written. And secondly, having backspace delete the whole word is a critical feature to avoiding annoyance. If none of the predictions are correct, I just tap backspace and then type the word like I usually would. This, combined with the rarity of those hiccups makes swipe typing quicker than regular typing to me.
It also helps with relaxing my wrists as I can also hold the phone with one hand and swipe with the middle finger of my other in a gesture like holding a pen, but without the pen.
Maybe there's actually a third aspect; screen and hand size. I haven't owned any big phones, my iphone 13 mini is the biggest phone I ever owned, but I do have large hands, so swipe typing with a single thumb is not very difficult. Your experience might be different depending on your anatomy and device.
