Any software besides the official software is unsupported and completely at your own risk whether that is on iPhoneOS or Maemo does not matter.

However, Extras is tested by this community and found stable, whereas the App Store is reviewed (but not tested in sense of QA!) by Apple and found stable, whereas Cydia is tested by community and found stable. One problem Cydia has is that with the scattered repositories you run a combination of repositories probably not well tested or taking each other into account.

Now, Extras-Devel is explicitly for development purposes. We now know that Eldar has installed software from Extras-Devel on the same device he is using to publicly judge stability of the N900 firmware. I question this behaviour. Extras-Devel is like installing an alpha/beta/rc firmware or OS. Much of the software has not been tested on the final product. Its like running Windows XP software on Windows Vista. Anything can happen.

I have seen iPhoneOS applications being reviewed by an iPhoneOS enthusiast news site, and they're usually a sneak preview. The reviewers are honest and informative in telling their experience both positive and negative wise without remarks like 'oh this sucks donkey' while also underlining they're not using the final version. Ie. "this bug was reported and confirmed" or "the bug is fixed in the final version currently pending reviewal". AFAIK Eldar does not commence such communication.