Sorry, i was a bit ambiguous. It's not the installing individual .debs that's the risk so much as installing apps from extras-devel. These are apps not expected to be fully ready for end users, so there are real problems you can run into. It doesn't matter whether you install the app from the repository or download the .deb and install it, either way, it's beta quality at best.

My point was, though, that if you're going to install beta quality software, at least do it from the repository like a sane person. This gives you the advantage of automatically resolving dependencies and keeping you informed when there are updates.