OP: It seems you'd like some actual evidence to go on, rather than just opinions. Me too, but it's awfully hard to come by. All I can offer is my conclusions.
I followed the reports of 1st-gen Time Capsules failing very closely here on MacRumors and a few web sites, because I've got one (a 500GB model purchased in May 2008). As you can imagine, I had good incentive to learn about the potential problems. I read nearly every thread about the TC for a couple of years, then tapered off to just reading them occasionally now. (This is an attempt to claim that I have some knowledge and long-term perspective.)
Anyway, what I eventually concluded/came to believe is that many of the 1st-gen TCs had defective capacitors in the (internal) power supply. Because of the defect, they failed at a lower heat than they should have. I distinctly remember seeing posts and pictures describing bulging and leaking capacitors. Failed caps caused the power supply to stop working, and the affected units would not even turn on or show a light. Importantly, the disk drive with its data was still fine and the data could be accessed by putting the drive into another enclosure/dock or by repairing the power supply (see below).
It was a fiasco, but Apple finally did release a list of serial numbers of (supposedly) the affected units and offered replacement, including data migration. (Mine fell within the "problem range" but never failed. It may well be that other units outside the serial number failed with the same problem.) There was no reason for anyone to lose their data, unless they refused to let Apple do it and refused to open the TC themselves and do it. Besides, the data on the TC was supposed to be a copy of data they had elsewhere.
Really, the 1st-gen 18-month death problem was analogous to an regular external enclosure failing, or, more precisely, its power supply failing. I read reports of people who had fixed their TC by replacing the defective capacitors. Someone was even performing the service for a fee. Others figured out what the voltage requirements were, and used an external power supply ("brick") wired into their TC. That also worked, reportedly.
Anyway, sorry I got so verbose. The real question is whether the current TC will have a "high" chance of failure. I believe it will not. I had continued reading TC posts on MacRumors for at least a couple of years after the 2nd-gen TC came out, and in my (fallible) memory, there there was no convincing pattern of posts describing failures of the later-gen TCs, as there had been for the 1st-gen ones. I didn't read the Apple Support forums, nor have I checked there now. Sure, there have probably been some problems, but a pattern of many experiencing a common problem with 2nd- to 4th-gen TCs? I haven't seen it on MacRumors, anyway.
I can give you my own experience, but that might not help much. I bought my 1st-gen TC in May 2008, it has a serial number in the range published by Apple, but it has never failed. I replaced the 500GB with a 2TB one in the fall of 2010 (I believe). It has been used constantly since 2008 as my TM backup destination, router, and WiFi AP. I've never had a problem with it and have been very happy with it.
Keep in mind that any enclosure can fail. (I just had a firewire enclosure fail on me -- disk is OK though.) As far as your backups are concerned, I think the TC has an equal or lower chance of failing than a typical HD in an enclosure.
Just my 2 cents. Oh -- and you can't beat the wireless backup capability if you've got portable computers! And as for a "cheap NAS", I don't think you can get anything as cheap as the TC. Good luck!
