Any USB2 drive is slow.
The 60MB/sec rate in the USB specs is the maximum burst rate, not the sustained transfer speeds. I’ve never seen a USB2.0 drive sustain over 30MB/sec, and depending on a variety of factors, it’s usually more around 20MB/sec. That would go for all USB drives, regardless of brand or manufacturer.
So yes, compared to your 100+MB/sec you get from the internal drive(s) in your PC, any USB drive will be slow, whether connected to a PC or to any other device. Writing from an attached drive, over the LAN to my PC, or vice versa, I usually see speeds around 10MB/sec, so the WDTV USB performance does appear to be less than with a PC, but it’s still in the same ballpark, since that’s all you may see on a PC under some conditions.
As far as I know (other than issues with drive courruption due to improper drive removal), the only issue with some Elements was due to the formatting, and re-formatting prior to use fixed the problem. I don’t really recall any specific issues with Passports or MyBook Essentials.
I have no personal experience, so I can’t comment on MyBook Live drives.
Pretty much any other manufacturer’s USB drive will have similar performance and restrictions to the WD ones, so you don’t gain anything there, but you don’t lose anything either.
As far as I know, when the original WDTV HD devices were shut down, the drive was “Safely Removed” and put into sleep mode, but you’d have to disconnect power from it to actually shut it off. I believe the Live, Live Plus and Live Hub all shut the drive down when powered off.