Good point. The Time Capsules at least early on even had a reputation for disk failure (from heat?).
I had a 1st gen TC that was fantastic. It was dead silent and ran TM perfectly on my MBP up until it died, about 18-24 months ago. When I did some research and corresponded with a few techs who repair them, I discovered my TC had a surprisingly long life compared to its brethren. The original design did cause the drives to overheat.
However, I understand the new design has addressed this and it is no longer an issue. Apple does learn from its mistakes and almost always fixes old problems in newer generations.
I was considering replacing my TC this year but I must use my Internet provider's wireless router, which makes the TC over-priced for my needs. I also decided I really need additional media storage and a bootable backup of my system (which I've been keeping on a portable drive). Localizing everything in one place, as well as finding a better media server, has led me in the direction of building a NAS. I've never built anything like that before, so I still haven't sorted exactly how I'm going to do it.
That being said, if you want a wireless external drive, need a WiFi router, and plan to use TM, the TC is an excellent solution. It runs seamlessly with TM, couldn't be easier to set up, and I've been told this generation is just as silent as the original. Once my TC was setup I never even thought about it unless I needed to access accidentally deleted or earlier versions of files. I loved and was heartbroken when it died.
In addition, the TC is supposed to have "server grade" hard drive, so essentially it should spin on the slower side (5400 RPM vs 7200, for example) and have firmware settings that address issues such as how frequently they park/unpark, etc.
There are different schools of thought, but in theory these settings and the design of the drive should make it more stable, to eliminate data loss/corruption, and extend its life. Remember, this drive I'd on 24/7 and working most of the time. A typical desktop drive, which is most likely what you'll find in most external hard drives, does not work nearly that much and the desire is generally for speed and performance. Different needs, different drives. According to those who subscribe to this idea, one would expect less stability, shorter drive life, and a higher failure rate from a desktop drive hooked up to the AE and using TM.
Then again, unless someone opens their TC, there's no way to know for sure if it really has a NAS/server hard drive or a regular old desktop drive.