Pure personal opinion here: frankly even aside from figure concerns, I feel they are a younger woman's look. A Hollywood star wearing the very top quality version can get away with a lot, but the average 40+ woman can probably find something... better.
Eta, looking at Diana's linen example I'm realizing this probably is due to my general discomfort/poison eye for very bare necklines. It's not even about cleavage as I'm ok with plunging necklines on say a wrap dress in the evening.

I hate them. I don't like wide open necklines, I like to wear a bra, and bias cuts are not friendly to my legs

Traditional slip dresses, in satin or silk and solid colours, are extraordinarily difficult to wear well. Any shapewear would show underneath, as would any clasp on a bra, not to mention any ripples in the skin, or any lumps or bumps you don't want highlighted. They are traditionally nightgowns for a reason. They are either very, very revealing or very, very, unflattering (or both) on most people.

Hated them even when I was young and firm. I need a bra with straps and nipple coverage and all that. Slip dresses are only for the bedroom, aka, nightgowns.I do like (and DH likes) how I look in a slip. I'm just saying..........not street wear, for me. Not even when I was 13 and had barely a chest. It's about the nipples more than the cup size.

I've been binge watching Friends on Netflix and can I tell you I love most of the fashions (it's been a hoot) but boy was there major nipplage.

Oh.. also.. without a bra my breasts and nipples are floating way too low for golden ration proportions these days!

I agree with everyone who says that they're impossible to wear and look fabulous unless you're built like a human clothes hanger.

They look better on bones than on fleshy flesh, and better on young bones, if we're being honest.

Best of the best would be young bones, minimal flesh, no undergarments, and a wearer who can tolerate a wardrobe malfunction or two. That leaves me -- and about 90% of the planet -- out.

This is a fun thread. Thanks everyone!