The check in the line is cursory, but there is a random secondary spot check for some once they get in stadium
https://buffalonews.com/news/local/about-1-of-bills-fans-referred-to-secondary-review-for-vaccination-proof-at-highmark-stadium/article_eb5b1ef0-2217-11ec-b679-bf3347823b9d.html
Also esp if it is raining would recommend doing the app. My nephew brought his card to the 10/3 game in the rain and it later disintegrated in his pocket (granted most people would take better care of it by why even worry about it if you can use the app)