Motors, generators and actuators are already used, so we know how to design for certification, nothing daunting there.
As has been pointed out previously the technology is already here. It's simply about the investment.
I agree that we're getting there, in terms of fly by wire, and criticality of electric components. However, when an electric motor replaces the very commonly accepted tail rotor driveshaft and gearboxes, the criticality will be perceived at a higher level. Let alone the unusual method of control (if it's even practical), the reliability of the motor system will have to be demonstrated to a new level. Such a design initiative will be labeled as novel and unusual, and held to an unusually vigorous standard. The present motors and generators (FBW notwithstanding) are certified as secondary systems whose failure can be managed by procedure. I perceive that investors would ask why they should invest an immense amount of money to certify a novel system which really only trades known and understood problems for unknown problems, without really solving any problems.
The project I was hired to advance toward certification for a motor powered 172 considered a purpose built 150HP electric motor. Though I saw designs and detailed drawings, the project never got to the point of producing a motor for installation (they took a lot of measurements though!). It was to be about 10% heavier than the Lycoming O-320 it would replace. The battery pack was a bit more of a challenge, though not insurmountable. Certification had a path forward with the authority, I had a number of discussions as to the proposed certification basis, and general agreement. That was doable - but it was a single engine airplane, where the failure mode was no worse (and really not much different) that the original design. I am confident this will happen for airplanes, it just requires a meeting of battery capacity, and airplane utility. It costs too much to keep a training airplane offline for hours to recharge it, and changing out very heavy batteries discharged for charged is problematic. During the planning of the 172 project, I did tell my client that they should install the motor as the primary power source in an R22, and not carry batteries, just a long power cord to the corner of the apron. Of course, you couldn't fly the R22 anywhere that way, but we spend a lot of time simply practicing hovering, so it could simply be a hovering trainer, which never gets higher than ten feet, nor leaves the apron. People liked the concept, but we did not get that far. Someone will.....
In the mean time, I'm very comfortable with shaft driven tail rotors/fans, we have more pressing product improvements to work on.