To clarify, extras-devel isn't guaranteed to brick your device, but there's every possibility that it will at any given time. Also note that it's unlikely (but not impossible?) anything in there will actually damage your device - it might just need a reflash. We used to have extras and extras-devel. The problem was that anything that wasn't ready for primetime, but was still reasonably safe got shoved in extras-devel, and then all the newbies would read about some cool beta apps, go add the repo, and then forget to disable it. Then, when they updated, all sorts of apps in stages of development ranging from "a bit buggy" to "kills kittens" would get installed, leading to a lot of complaints and general unhappiness. This issue was talked out last year in the community, and lead to what we have now. Extras is safe. Extras-testing is not guaranteed to work properly, but should be reasonably safe as well, and is intended for developers to get feedback from early adopters. Extras-devel is essentially just a mish-mash of whatever people need to throw into a repo, and may work, or be horribly broken.
Thus, extras-devel is not something you should use unless you know what you're doing, and have no other option. However, if you absolutely must install something from it, then add the repo, install the app, and disable/remove the repo immediately. If that app happens to do Bad Things, tough luck, but the bigger danger is leaving it enabled, as the updater just checks for a higher version number, not for some label designating if the app is stable or not.

To get back to your original question, both the App Manager, and the command line tool apt-get automatically resolve dependencies. Dependencies are, well, things that the app depends on. For example, if lots of apps need some particular mathematical calculation, then someone will code that, and other apps will just tell the App Manager that they need that bit of code as well. By not bundling everything all together, you can save a lot of space, however, you have to install all the dependencies before you can install the app you want. dpkg installs one or more specified packages from local files, but doesn't do the resolution step for you. If you try to install something with dpkg, it will tell you what else it needs (e.g. libsoup). To do things the hard way, you would then go find libsoup, try to install it, and if it needs something else, just repeat. This is what people used to have to do "In the Old Days", and was referred to as Dependency Hell. So essentially, your only easy option in this case is to add the repo. I know how tempting it is to try all the latest and greatest apps before they're ready, but the thing is -- they're not ready yet. If you've read all this, and you want to try it, we won't stop you, but if you break something and can't figure out how to fix it, you're likely to get a "told you so" response.
Cheers,