Basically, this episode would fit in much better in the Twilight Zone universe than in the Black Mirror universe. When you watch it, keep an open mind and think of the Twilight Zone instead.
The concept is an interesting one that reminds me of Nope and capturing the spectacle, and the supernatural element is FUN! It's just that Black Mirror has not been a supernatural show and so when the *thing* happens your immediate reaction is something along the lines of "whaaat is going on?!?!" When you step back and look at the episode by itself, it's fun and entertaining with a great twist that I certainly didn't expect! It's not the greatest episode of Black Mirror (or Twilight Zone, for that matter) but in my opinion, it's worth more than the ratings it's getting, and is worth the watch.