My lightweight steel touring bike carries 40lbs easily in four panniers but starts getting noodley at 50lbs and beyond. My mountain bike and tandem can each handle about 10lbs more before getting interesting and are both better behaved when overloaded.
I have ridden a Bakfiets with my sister-in-law in the bucket and it handled that load excellently. I have never used one for groceries but It seems like it would be more convenient to just dump everything in the bucket of a Bakfiets than distribute the load to panniers.
I have never ridden a longtail cargo bike but am not convinced that it would be better in any way than a Bakfiets.