[liability]

I'm confident you can construct an unhealthy taco, but is there anything inherently wrong with tacos? Fundamentally it's just a bit of meat, veggies and cheese. The tortilla is grain and thus arguably not great, but tortillas are pretty small compared to the average sandwich bread.

[bluGill]

Sandwich bread is mostly air, tortillas are smaller, but they seem to contain a lot more substance.

[CameronNemo]

Tortillas are made from nixtamal, which has some nutritional improvements over raw/heat-only corn.

https://en.wikipedia.org/wiki/Nixtamalization