The best Onigiri are the ones that you nigiru yourself, homemade. Konibini Onigiri are convenient in a pinch, but they pale in comparison to homemade ongiri from some who know the right nigirikata techniques. Plus, homemade has way more variety, the choices of what you can put in them and mix with them are endless, furikake is great for making ongiri. I wouldn't consider it a comfort food, I think it would be more correct to compare it to the likes of a sandwich in many Western countries.
I agree, they most closely resemble sandwiches from Western cooking. I think Konbini ones are just bland most of the time. A few of the super markets that make their own seem to taste better in my opinion than the Konbini ones. I do think the onigiri with tempura on the inside is a real treat. However, sometimes I want just plain salted rice ones if I am in a rush and eating pre-made super market foods.
