Actually I never had any problems with packges from the devel repositories though I am constantly using it for apps which are not in testing but raised my interest.
Note that lots of apps ported from the desktop are not optified yet while the ones written from scratch often already are. That means that they will fill your small root file system with data and binaries thus quickly overloading it. In certain cases it is even hard to get rid of those after they have been optified some day.
As a second issue programs might require higher versions of libraries which might then be incompatible with the ones already provided. If such libraries are installed they can ruin your installation.
However operating with care helps here and reducing installation of apps from devel repositories to the mininum increases the chances of keeping everything in order.
As I said I never had issues with extras devel but be prepared for the worst case according to Murphies Law