The trendy croissant sandwich is built on a bread that fights being a sandwich. A croissant is laminated dough, dozens of butter layers folded and baked into a flaky, hollow, fragile shell, and it shatters and slides apart under the weight and moisture of a filling. The whole sandwich is an argument with that structural problem, and the bakeries that made it a format treat the croissant's richness and collapse as the feature, not a flaw to engineer around the way a sturdy sub roll is.
The craft is in managing the fragility. The croissant is split horizontally and the filling kept deliberately light and not too wet, because a heavy or saucy load drives straight through the layers and turns the pastry to grease-soaked paste. It is often warmed so the butter softens and the crumb pulls together enough to hold, then filled with things that complement rather than overwhelm: ham and a melting cheese, egg, or a cured fish and a soft cheese. The shell does flavor work a neutral bread never would, so the build leans on contrast with that butteriness rather than piling volume into it. The bakery-counter reality is a case of fresh croissants split and filled to order, kept simple because the bread is the expensive, laborious part and the point.
The most recent turn is the Korean-influenced cube and cream-filled style, which pushes the form toward dessert with sweet fillings in a reshaped pastry, and the savory artisan build that treats the croissant as a serious sandwich vehicle is its own line. These adjacent forms are their own sandwiches and deserve their own articles rather than being crowded in here.