1. Linux kernel "mem-sleep" mode default [0], which determines what actually happens when systemd-sleep "suspend" is called. The default value is "s2idle" for many distros which is as good as leaving your computer running with processes paused... when what most users expect is some kind of suspend-to-ram ("deep"). Obviously this burns through battery pretty quickly but at least it shouldn't overheat.
2. I found this systemd infinite hybernate loop bug [1] (now fixed) when swap is smaller than RAM... although you only would come across this is you accidentally under-provisioned the swap partition on install like I did - but the failure mode was spectacularly bad, because it will retry over and over swapping and compressing the RAM which obviously ramps up the CPU, get hot with the lid closed, make short work of battery and SSD.
Anyway the common issue is #1 which you just have to know about otherwise your computer doesn't really suspend at all, and since most machines are very quiet these days you won't notice unless you dig.
[0] https://unix.stackexchange.com/questions/538541/suspend-to-r...