Arch Linux has a decent guide explaining the behind-the-scenes stuff: https://wiki.archlinux.org/index.php/Power_management/Suspen...

On desktops and laptops, it Usually Should Just Work (tm). The general culprits for issues are proprietary graphics and wifi drivers as well as shoddy BIOSes with broken ACPI tables, but I never had to dig this deep.

For embedded systems, the situation is more complex, as the chipset needs to support it - suspend itself is not the problem in most cases, but wakeup is - for example, the Raspberry Pi does not have a dedicated interrupt (https://github.com/raspberrypi/linux/issues/1281). If you're working with an embedded chipset, your best bet is the manufacturer, they have to deal with that in the BSP.

Whenever I read comments like this, I think "Why is he trying to use a 10+ years old Ubuntu, or similar"? Or do I just make way luckier buying choices? Suspend to anything has worked flawlessly out of the box for my devices since 10+ years. Do you have a specific example of what is not an "obviously bad hardware combination choice", but yet still does not correctly support Suspend-to-RAM?