I don't own and haven't run the USBPre 2, just forwarding along info I've heard and read on it. There are a lot of posts on the USBPre here, best to do a search on it to dig up the specific posts. I've heard about the A/D architecture and preamp sections second hand through a SD reseller, supposedly coming from SD, and also have read the same thing on ts.com -- again, information supposedly coming right from SD. FWIW, YMMV, etc, etc.
The noise performance on the USBPre isn't as good as the 722, but it is pretty close. At that point, it might not mean different preamp sections or architecture and may just be getting into the effects of board layout, etc. The preamp sections could be very different, I'm just saying that the noise specs themselves may not be a reliable means of telling whether they're different.
On the D50, I've owned one for years and find it to be pretty solid, esp for a handheld recorder. Not to the level of the 702 or 722 (I've owned both of those -- just built like tanks). Then again, I wouldn't take much stock in the fact that there are broken D50's out there. The 702/722 units break too (though at the price of buying one, those are probably something someone gets fixed and doesn't just dump them). The LEDs on the 722 are known to go out, and I've heard of the main board on 722 units getting fried. Everything can break, a couple anecdotal data points on failure don't mean much -- though again, I agree that the 7x2 units are very, very robust.
I don't think there is much arguing that for field work and nature recording, the 702/722 may be your best bet, and with used prices you can get one for not that much of a premium over the USBPre+D50 combo, esp since the USBPre is just out and you won't likely find those used. But a USBPre+D50 combo can get a lot of the audio benefits of a 722, and will cost several hundred less.
One final note: The D50 can be finicky about getting a digital input -- the sending device must follow the SPDIF standard exactly, and many A/D's don't follow the standard exactly, esp regarding consumer bits. I haven't followed the USBPre threads enough lately since I decided to get a Lunatec V3 instead. You should dig around to make sure the USBPre plays nice with the D50 if you want to go this route.