I used Linux at work for years. Sleep just works, external monitor also just works. HiDPI was rough at the start but works fine now.
Touchpads do kind of suck. I generally really dislike the default mouse acceleration. Font rendering is still so so if you don't have a HiDPI screen and the most popular desktop environments are still kind of terrible.
But sleep definitely does work.
Are you implying that those users are lying?
I'm sure sleep does work reliably for them.
'Does sleep work on linux' is a fallacious question to begin with, because sleep working/not working depends on the hardware.
On some configurations it works flawlessly, on others it doesn't. Therefore you will always have some people saying it works, and others saying it doesn't. FWIW, my current laptop is a machine that ships with linux (system76 darter pro) and sleep works 100% reliably.
In my experience, when sleep doesn't work reliably, it's usually due to buggy firmware behaviour because most vendors don't care about supporting anything other than windows.
Along those lines, since most OEMs don't ship/test linux, it's simply not possible for every single hardware configuration to work flawlessly with linux.
Whenever you bring anything like this up though you'll just get a load of "When was the last time you tried it? It works perfectly for me" replies. Linux users don't want to admit its flaws.