Hibernation has been getting better, yes.

I especially like both the biggest updates to the tech, on Windows: partial snapshot, instead of full snapshot (which means less writing to disk, particularly nice for SSDs, and allows for faster sleep/resume cycles), and hybrid sleep (suspension with simultaneous writing of the hibernation file, meaning you can resume from a suspended PC that lost power).

Also, it's good to point out that since W8 "power off" is actually a variation on the snapshot hibernation, the OS doesn't need to unload and reload itself again. Current system status is written to disk, and loaded on power up. Only a reboot will actually reload everything, and that only happens when Windows Update needs to update system files, or when you reboot to UEFI/OS manually. Which is why W8 manages "instant on" and "active standby" scenarios (it basically cheats :P).

As for specific issues, well, so far I have had zero with W8, when it comes to the standard "power down" option. Hybrid sleep and hibernation, however, both on W8 and previous, do sometimes have quircks, and hang, unable to restore properly. No ill effects stemming from just deleting the hibernation snapshot and rebooting, though.

As for the issues of "stuff being changed in RAM" and Windows becoming slower, two things:

1) When you issue the hibernation command, all commands to and from software are frozen, and written to disk. If something happens to any piece of software upon coming back from hibernation is because it's either not hibernation-friendly (some apps can sense connectivity changes, and freak out, for instance, or depend on something else that may or may not be 100% working when they get reloaded) or because something happened when writing to the file (or reading from it). Random read and write errors do occur from time to time, after all.

2) Windows has a natural tendency to get sluggish after long uptime periods. RAM might not get properly flushed, old code might be lingering, there might be a memory leak somewhere, etc. Point is, Windows slowing down doesn't come from hibernating, it comes from a long uptime. And even then, these days it usually takes a LONG time for that to happen. Like at least a week or more of uptime before you start feeling something is off.