It's probably a bit harsh to conclude the new M4C will be comparatively worthless after the ROF nerf, but I agree it certainly lost a lot of its appeal over the other premium med options the soviets can field. I'd wish the 76mm could get at least a bit of its ROF back via Vet to make up for the drop in both AI and AT performance. That way it wouldn't be too oppressive right out of the gate and keeping it alive would be more rewarding.
Another thing that could make the soviet M4C more interesting would be some sort of compensation for not having access to on-the-field repairs through vehicle crews like its USF counterpart has. Obviously soviet vehicle crews are out of the question, but what if the M4C could get a vehicle repair ability, maybe gated behind Vet 1 if needed, to make up for this?
As for the secure mode ability, I'd go for either improving cap/decap rate as suggested before, or just allow the hull/coax MGs to fire while it's active. Right now it feels highly situational and apart from maybe blocking a last-minute VP capture attempt I haven't seen anyone make good use of it in a while.
The idea that unit shared from two faction should be identical is flawed. If that was the case Ostheer Puma/Panzer IV should have 5 vet level.
If there is a need one can change the price.
When it comes to Soviet M4 it is already has smoke over other Soviet tank and it is also easier to benefit from radio net than USF one.
As for the unit I would rather have cost the same a normal Sherman and adjust stat so that USF unit combined the two unit.