Spot-checking some of the descriptions cut off in the under-200 character range, we realized that a number of them were video snippets, which seem to have shorter limits:

https://d1avok0lzls2w.cloudfront.net/uploads/blog/meta-desc-2017-5-36976.png

These snippets seem to generally max out at two lines, and they're further restricted by the space the video thumbnail occupies. In our data set, a full 88% of video snippets were cut off (ended in "..."). Separating out video, only 32% of organic snippets (removing the video results) were cut off.