If you charge two halves of a muscle oppositely so they attract, you would get arcing between them without a strong insulator in between. But if you charge two halves both negative or both positive there would be no arcing between them. They would still strongly repel and it seems like you could use that to do work.
Electrostatics generally require high voltage (the HASEL muscles you linked to require 2-20kV [0]) to generate usable force.
This creates lots of engineering and regulatory challenges for product development because, as the esteemed William Osman says, "Welcome to the world of high voltage, where everything's a wire and you're probably going to die" [1].
Even if you do not have a motor but just a single charged sphere, at its surface there will be an electric field increasing with the charge.
At a certain charge value, and at a not very large one, the surface electric field will exceed the breakdown field of air and a corona discharge will start.
So unfortunately the idea of using the repulsion forces does not work.
To prevent the discharges, the entire environment around the 2 charged moving parts would have to also be charged with the same sign, but then the repulsion forces between your parts would be balanced by the repulsion forces from the surrounding medium.
High-voltage equipment, e.g. high-voltage transformers, is indeed immersed in insulating fluids, either in special insulating oils or in the gas sulfur hexafluoride, which is more convenient than liquid oil.
This increases several times the breakdown field compared to air but it is not enough to reach similar energy densities like with magnetic fields.
To give some numbers, the air breakdown field is around 3 MV/m, while the maximum magnetic field in a motor might be up to 2 Tesla.
The ratio of the energy densities is the square of the ratio between the product of the magnetic field with the speed of light and the electric field, i.e. the square of (2 x 3 x 10^8) / (3 x 10^6), so 200 squared, i.e. 40 thousands.
Even if you increase the breakdown field 10 times, which is quite hard to achieve with fluids, the magnetic field would give forces much, much higher at a given size.
For a bit, but if you keep raising the voltage you find the electric breakdown happing on the outside surface of the robot (or whatever’s using the electric field muscle) instead of where you put the insulation.
You can't create a monopole, either electrostatic or magnetic. If you impress a negative charge on something, there must be an equally strong positive charge somewhere nearby that you'll have to insulate somehow.
