From personal experience with a Neurotrac Pelvitone, does it help with PFEs? Yes, but the effectiveness depends on the electrode type and voltage being used. As noted above, physiotherapists are reluctant to recommend the use of such a device unless there is a minimal risk of cancer recurrence, i.e. as far as can be assessed, effectively zero.
Do PFEs help with regaining continence, whether or not electrically stimulated? Again from personal experience, if they are going to help, they will, if they are not going to help, they won't. Pelvic floor strength is only one factor governing the recovery of continence.