Never heard of the EMsella chair. Did a bit of reading and what I found was a mixed bag. Essentially similar treatments have been around for over a decade although the BTL EMsella device is fairly new to the market. As far as I can tell there has been only one study of electromagnetic fields treatment for incontinence in men after prostate surgery.
Reducing urine leakage after radical retropubic prostatectomy: pelvic floor exercises, magnetic innervation or no treatment? A quasi-experimental study.Most of the studies I have looked at seem to suggest that the treatments seem to work somewhat for women with stress incontinence but the improvements don't appear to last long after the end of treatment.
Here's a fairly typical review study from a couple years back.
Efficacy of electromagnetic therapy for urinary incontinence: A systematic review.Their conclusion:
There is no firm evidence to support the benefits of using MS in the management of UI, although short-term outcomes suggests that MS improves UI symptoms in women. The applicability of MS as a treatment option for UI remains uncertain until larger, high-quality trials with longer follow-up periods using comparable and relevant outcomes are conducted.Note that "no firm evidence to support the benefits" is not the same thing as "firm evidence there are no benefits". It's just that the studies have been small, and poorly structured, and the results somewhat all over the map.
There's no reason not to try the EMsella chair if your insurance covers it and/or it's not too expensive. Similar treatments have had excellent safety records and some people in some studies seem to find it helpful.