I have wondered about the same thing. With an electric system, it should be negligible extra costs, so that part is not an issue to me. Rather, the extended mash time could seemingly alter the pH as the grain is “re-rinsed” over an extended time period following conversion. I have no science on that, but it stands to reason that the active heating of the mash combined with the amount of liquid overall passing through the grainbed might be a source of problems, at least potentially. I would say it is ripe for an experiment!
Other concerns are evident with an unattended process- pump failure, overheating the herms tun (boil out), etc...but you might be able to set alarms for that.