Modern Standby enables much faster return from Standby. You open the lid of your device and it's there. That's not the case with S3. I find it adds a lot of convenience.
My shiny new work laptop without S3 wakes up in the same time, and then says "Battery critically low, shutting down."
This is not a convenience, it's insubordination. I told it to sleep, and it didn't.
But you know what's really insane? When the new work laptop is just sitting there with the screen locked and I walk up and hit a key, it takes longer to just display the password box than the 10 year old Thinkpad takes to wake up from S3 and display the password box. (Ok, it's an unfair comparison, Windows vs Linux, but still...)
Modern standby is making my laptop unpredictable, and that is very inconvenient for me
How is this the case? Suspend to/from RAM on Linux wakes up in less than a second in my experience. Which makes sense, because everything is still in RAM and systemd doesn't need to do a bunch of work to get everything back online. Maybe Windows has a weird implementation of S3?
And S3 is basically «instant».
So I don't see much point in S2Idle/S0ix.