YMMV, yes I agree that the convini sandwich is something to try and the perfect thing for a quick "oops I forgot to prepare anything" picnic in Yoyogi Park.
However my main critique is that some times their filling is just a small line across the bread, where the cut showing the contents is, and the rest is basically empty bread. You can see an example of what I mean in this article (it's about Taiwan, but in Japan I've seen way worse): http://jonathanlee.org/2014/02/21/interlude-treachery-7-11-s...
You can also guess it from this side-view; yes the meat is just a strip along the cut edge, the rest is stale-ish bread: https://grapee.jp/en/wp-content/uploads/s-50126_05.jpg
It depends on the type of sandwich, so it goes from "always wrong" (like the last pic) to "it really depends" on some others. Normally it's okay though: https://soranews24.com/2020/10/24/25-different-japanese-conv...