I have decent experience using my own FDM printer and third party services using SLS, I design my own mechanical parts too. I know what you mean and I know joy and frustrations of owning a 3d printer. There’s no arguing that quick turnaround helps design process tremendously. It’s just currently I have to make do without one.
They (JLC) have requirements well specified and automatic rule checker rejects problematic parts. I had several virtual iterations with it before I got my parts dialed in. They should cause no problems.
As for them restarting failed prints, heh. I really hope not. Large machines tend to be loaded to the brim with multiple jobs combined and they are pretty reliable.