The displaying of video thumbnails on SMS campaigns is a highly variable situation.
The reality is that different devices and carriers behave differently with respect to displaying a thumbnail image (alongside a video URL). We’ve noticed that Apple devices on the larger carriers (top 5) have a better chance of displaying a thumbnail, whereas Android phones and pre-paid carriers are less likely to display a thumbnail.
We recommend writing the copy in such a way that it works in both situations (with or without a thumbnail image).