I have an Ariel Rider C class, which is a step through. It has a built in (or on?) rear rack, to which I've added a top trunk and folding baskets on each side. The top trunk never carries much weight. I regularly load the folding baskets up with groceries which can be pretty heavy--beer, milk, etc. Often, the extra weight is all on one side, and I never notice any problem whatsoever from that. I have never noticed any sway or wobble that seemed to result from the step through configuration. In fact, there's not much movement at all in the frame. The battery is on the front of the seat tube, making the frame resemble one of Rad's bikes (don't know the model name). That puts the weight low and centered. Also has a mid motor. I do carry a bag on the handlebars, but it doesn't carry much weight. I have never had the front wheel threaten to lift, even though this is a very short wheelbase bike with an upright riding position (which I really like, as my wrists and hands used to get painfully numb after some distance).
I like the step through because on my former non-electric Trek diamond frame, swinging my leg over with the rear rack loaded was extremely awkward. Whenever a sassy kid says, "Hey boomer, how come ya riding a girl's bike?" I answer, "This ain't no girl's bike kid, this here's a STEP THROUGH! Besides, I'm too old to qualify as a boomer."