Sweet potato pie is very similar to pumpkin pie - imagine mashed sweet potato made a bit sweet, add cinnamon and creaminess, and the marshmallow is basically like meringue once cooked. It can be very nice with a sort of treacle flavour.
Or, like many recipes, it can easily be done badly.
The reason they say use canned pumpkin for pumpkin pie is that here we only have pumpkins grown to be Jack o'lanterns, so there's not that much flesh and it's not very flavoursome. Still edible and I roast it and the seeds each year, but nothing like a good eating pumpkin.
My mum's family are American so I grew up eating excellent Midwest American food (meatloaf with chunks of gherkin, piles of buckwheat pancakes with bacon and eggs, many pies, steak, slaw, etc).
And learned to cook with mum's English adaptations and a 50s Betty Crocker (sort of fictional American Delia). Mum bought me my own copy when I left home, only to find it had been updated for the 80s - not only calorie counts on everything and low-cholesterol options, but huge move to reliance on processed ingredients - to be fair, mushroom soup is used like cartons of bechamel sauce here, and given the existence of flavoured packets of rice, gravy granules, and pancake mix here, the UK is hardly in a position to talk. But this book infuriated Mum who went to look up a recipe for deviled ham and all of them said 'stsrt with 1 can deviled ham...'
American cookbooks do tend to have more focus on preserves and canning, despite huge freezers - this made sense when I stayed with my aunt during harvest and it's a rural area, 1 hour drive to The Store, but people would put out signs with what they could sell - like rural UK, only it would be by the bushel. Two bushels of tomatoes is 70 litres.
Mum remembers jello salads and molded salads all too well. Her theory is a) people wanted to show off their new Tupperware shapes (real Tupperware is brilliant, we're still using hers from the 50s), and b) it was the 60s and everyone was stoned.
Or at least the creators of those recipes.