I can't speak to the electrical part with any authority. But mechanically, sharp edges are stress risers. Thermal or mechanical fractures will form there much easier than if it's a smooth radius. Think of it like a piece of paper. It's much easier to tear it on the edge than pull it apart from the center. In terms of vaping, 3xx series stainless,kanthal and ni-crome don't harden. That alleviates some of that issue. The harder a material the easier it is to crack.
Before the trolls come out yelling about how SS 316 hardens, I mean this in the metallurgical sense. 3xx stainless is a non hardening alloy. It gets harder but not what most people that work with metal consider hard.
As far as it being a coil it's a design choice as much as anything. A good choice for the design problem but not the only solution. If you look around you find a couple other solutions. Two heating bars that wick lays between, and SS mesh in various configurations(including coils) are some off the top of my head.